diff --git a/.changeset/few-clouds-play.md b/.changeset/few-clouds-play.md
new file mode 100644
index 0000000000..d57ab69734
--- /dev/null
+++ b/.changeset/few-clouds-play.md
@@ -0,0 +1,5 @@
+---
+"@siteimprove/alfa-style": minor
+---
+
+**Added:** CSS shorthand property `mask` and corresponding longhand properties is now supported.
diff --git a/packages/alfa-style/src/property/helpers/match-layers.ts b/packages/alfa-style/src/property/helpers/match-layers.ts
new file mode 100644
index 0000000000..3152284d09
--- /dev/null
+++ b/packages/alfa-style/src/property/helpers/match-layers.ts
@@ -0,0 +1,39 @@
+import { List, type Value } from "@siteimprove/alfa-css";
+
+import type { Style } from "../../style.js";
+
+/**
+ * {@link https://drafts.fxtf.org/css-masking/#layering}.
+ *
+ * @remarks
+ * The computed value depends on the number of layers.
+ * A layer is created for each of the comma separated values for `mask-image`.
+ *
+ * If there are more values than layers, the excess values are discarded.
+ * Otherwise, the values must be repeated
+ * until the number of values matches the number of layers.
+ */
+export function matchLayers<V extends Value>(
+  value: List<V>,
+  style: Style,
+): List<V> {
+  const numberOfLayers = Math.max(
+    style.computed("mask-image").value.values.length,
+    1,
+  );
+
+  const numberOfValues = value.values.length;
+  if (numberOfValues === numberOfLayers) {
+    return value;
+  }
+
+  return List.of(
+    (numberOfLayers < numberOfValues
+      ? value.values
+      : Array(Math.ceil(numberOfLayers / numberOfValues))
+          .fill(value.values)
+          .flat()
+    ).slice(0, numberOfLayers),
+    ", ",
+  );
+}
diff --git a/packages/alfa-style/src/property/mask-clip.ts b/packages/alfa-style/src/property/mask-clip.ts
index a2b08e9c03..e05318d69a 100644
--- a/packages/alfa-style/src/property/mask-clip.ts
+++ b/packages/alfa-style/src/property/mask-clip.ts
@@ -2,14 +2,17 @@ import { Parser } from "@siteimprove/alfa-parser";
 import { Box, Keyword, List } from "@siteimprove/alfa-css";
 
 import { Longhand } from "../longhand.js";
-import { matchLayers } from "./mask.js";
+
+import { matchLayers } from "./helpers/match-layers.js";
 
 const { either } = Parser;
 
 type Specified = List<Box.CoordBox | Keyword<"no-clip">>;
 type Computed = Specified;
 
-export const initialItem = Keyword.of("border-box");
+export namespace MaskClip {
+  export const initialItem = Keyword.of("border-box");
+}
 
 /**
  * {@link https://developer.mozilla.org/en-US/docs/Web/CSS/mask-clip}
@@ -17,7 +20,7 @@ export const initialItem = Keyword.of("border-box");
  * @internal
  */
 export default Longhand.of<Specified, Computed>(
-  List.of([initialItem]),
+  List.of([MaskClip.initialItem]),
   List.parseCommaSeparated(either(Box.parseCoordBox, Keyword.parse("no-clip"))),
   (value, style) => value.map((value) => matchLayers(value, style)),
 );
diff --git a/packages/alfa-style/src/property/mask-composite.ts b/packages/alfa-style/src/property/mask-composite.ts
index 853b176270..bb7e80a783 100644
--- a/packages/alfa-style/src/property/mask-composite.ts
+++ b/packages/alfa-style/src/property/mask-composite.ts
@@ -1,7 +1,8 @@
 import { Keyword, List, type Parser as CSSParser } from "@siteimprove/alfa-css";
 
 import { Longhand } from "../longhand.js";
-import { matchLayers } from "./mask.js";
+
+import { matchLayers } from "./helpers/match-layers.js";
 
 export type CompositingOperator =
   | Keyword<"add">
diff --git a/packages/alfa-style/src/property/mask-image.ts b/packages/alfa-style/src/property/mask-image.ts
index fd52551ba7..7111fb511c 100644
--- a/packages/alfa-style/src/property/mask-image.ts
+++ b/packages/alfa-style/src/property/mask-image.ts
@@ -6,10 +6,12 @@ import {
   URL,
   type Parser as CSSParser,
 } from "@siteimprove/alfa-css";
+import { Selective } from "@siteimprove/alfa-selective";
 
 const { either } = Parser;
 
 import { Longhand } from "../longhand.js";
+import { Resolver } from "../resolver.js";
 
 export type MaskReference = Keyword<"none"> | Image | URL;
 
@@ -33,5 +35,14 @@ type Computed = Specified;
 export default Longhand.of<Specified, Computed>(
   List.of([MaskReference.initialItem], ", "),
   List.parseCommaSeparated(MaskReference.parse),
-  (value) => value, // TODO: How to resolve absolute URL?
+  (value, style) =>
+    value.map((images) =>
+      images.map((image) =>
+        Selective.of(image)
+          .if(Image.isImage, (image) =>
+            image.partiallyResolve(Resolver.length(style)),
+          )
+          .get(),
+      ),
+    ),
 );
diff --git a/packages/alfa-style/src/property/mask-mode.ts b/packages/alfa-style/src/property/mask-mode.ts
index 684d9d744e..56cd0f4136 100644
--- a/packages/alfa-style/src/property/mask-mode.ts
+++ b/packages/alfa-style/src/property/mask-mode.ts
@@ -1,7 +1,8 @@
 import { Keyword, List, type Parser as CSSParser } from "@siteimprove/alfa-css";
 
 import { Longhand } from "../longhand.js";
-import { matchLayers } from "./mask.js";
+
+import { matchLayers } from "./helpers/match-layers.js";
 
 export type MaskingMode =
   | Keyword<"alpha">
diff --git a/packages/alfa-style/src/property/mask-origin.ts b/packages/alfa-style/src/property/mask-origin.ts
index 4e13219ad6..de1e3f125e 100644
--- a/packages/alfa-style/src/property/mask-origin.ts
+++ b/packages/alfa-style/src/property/mask-origin.ts
@@ -1,12 +1,15 @@
 import { Box, Keyword, List } from "@siteimprove/alfa-css";
 
 import { Longhand } from "../longhand.js";
-import { matchLayers } from "./mask.js";
+
+import { matchLayers } from "./helpers/match-layers.js";
 
 type Specified = List<Box.CoordBox>;
 type Computed = Specified;
 
-export const initialItem = Keyword.of("border-box");
+export namespace MaskOrigin {
+  export const initialItem = Keyword.of("border-box");
+}
 
 /**
  * {@link https://developer.mozilla.org/en-US/docs/Web/CSS/mask-origin}
@@ -14,7 +17,7 @@ export const initialItem = Keyword.of("border-box");
  * @internal
  */
 export default Longhand.of<Specified, Computed>(
-  List.of([initialItem], ", "),
+  List.of([MaskOrigin.initialItem], ", "),
   List.parseCommaSeparated(Box.parseCoordBox),
   (value, style) => value.map((value) => matchLayers(value, style)),
 );
diff --git a/packages/alfa-style/src/property/mask-position.ts b/packages/alfa-style/src/property/mask-position.ts
index 9df75f5adb..1dc321769b 100644
--- a/packages/alfa-style/src/property/mask-position.ts
+++ b/packages/alfa-style/src/property/mask-position.ts
@@ -6,16 +6,19 @@ import {
 } from "@siteimprove/alfa-css";
 
 import { Longhand } from "../longhand.js";
-import { matchLayers } from "./mask.js";
 import { Resolver } from "../resolver.js";
 
+import { matchLayers } from "./helpers/match-layers.js";
+
 type Specified = List<Position>;
 type Computed = List<Position.PartiallyResolved>;
 
-export const initialItem = Position.of(
-  Position.Side.of(Keyword.of("left"), LengthPercentage.of(0)),
-  Position.Side.of(Keyword.of("top"), LengthPercentage.of(0)),
-);
+export namespace MaskPosition {
+  export const initialItem = Position.of(
+    Position.Side.of(Keyword.of("left"), LengthPercentage.of(0)),
+    Position.Side.of(Keyword.of("top"), LengthPercentage.of(0)),
+  );
+}
 
 /**
  * {@link https://developer.mozilla.org/en-US/docs/Web/CSS/mask-position}
@@ -23,7 +26,7 @@ export const initialItem = Position.of(
  * @internal
  */
 export default Longhand.of<Specified, Computed>(
-  List.of([initialItem], ", "),
+  List.of([MaskPosition.initialItem], ", "),
   List.parseCommaSeparated(Position.parse(/* legacySyntax */ true)),
   (value, style) =>
     value.map((positions) =>
diff --git a/packages/alfa-style/src/property/mask-repeat.ts b/packages/alfa-style/src/property/mask-repeat.ts
index 9b3c5def10..55504d02d6 100644
--- a/packages/alfa-style/src/property/mask-repeat.ts
+++ b/packages/alfa-style/src/property/mask-repeat.ts
@@ -2,7 +2,8 @@ import { Keyword, List, type Parser as CSSParser } from "@siteimprove/alfa-css";
 import { Parser } from "@siteimprove/alfa-parser";
 
 import { Longhand } from "../longhand.js";
-import { matchLayers } from "./mask.js";
+
+import { matchLayers } from "./helpers/match-layers.js";
 
 const { either } = Parser;
 
diff --git a/packages/alfa-style/src/property/mask-size.ts b/packages/alfa-style/src/property/mask-size.ts
index 993729084e..d00337da67 100644
--- a/packages/alfa-style/src/property/mask-size.ts
+++ b/packages/alfa-style/src/property/mask-size.ts
@@ -8,7 +8,8 @@ import { Parser } from "@siteimprove/alfa-parser";
 
 import { Longhand } from "../longhand.js";
 import { Resolver } from "../resolver.js";
-import { matchLayers } from "./mask.js";
+
+import { matchLayers } from "./helpers/match-layers.js";
 
 const { either } = Parser;
 
diff --git a/packages/alfa-style/src/property/mask.ts b/packages/alfa-style/src/property/mask.ts
index a412d0f60b..5007d3b0fa 100644
--- a/packages/alfa-style/src/property/mask.ts
+++ b/packages/alfa-style/src/property/mask.ts
@@ -3,7 +3,6 @@ import {
   List,
   type Parser as CSSParser,
   Position,
-  type Value,
   Box,
   Keyword,
 } from "@siteimprove/alfa-css";
@@ -13,16 +12,14 @@ import { Option } from "@siteimprove/alfa-option";
 
 import { Shorthand } from "../shorthand.js";
 
-import type { Style } from "../style.js";
-
 import { MaskReference } from "./mask-image.js";
 import { BgSize } from "./mask-size.js";
 import { RepeatStyle } from "./mask-repeat.js";
 import { CompositingOperator } from "./mask-composite.js";
 import { MaskingMode } from "./mask-mode.js";
-import { initialItem as posInitialItem } from "./mask-position.js";
-import { initialItem as clipInitialItem } from "./mask-clip.js";
-import { initialItem as originInitialItem } from "./mask-origin.js";
+import { MaskPosition } from "./mask-position.js";
+import { MaskClip } from "./mask-clip.js";
+import { MaskOrigin } from "./mask-origin.js";
 
 const {
   doubleBar,
@@ -35,42 +32,6 @@ const {
   separatedList,
 } = Parser;
 
-/**
- * {@link https://drafts.fxtf.org/css-masking/#layering}.
- *
- * @remarks
- * The computed value depends on the number of layers.
- * A layer is created for each of the comma separated values for `mask-image`.
- *
- * If there are more values than layers, the excess values are discarded.
- * Otherwise, the values must be repeated
- * until the number of values matches the number of layers.
- */
-export function matchLayers<V extends Value>(
-  value: List<V>,
-  style: Style,
-): List<V> {
-  const numberOfLayers = Math.max(
-    style.computed("mask-image").value.values.length,
-    1,
-  );
-
-  const numberOfValues = value.values.length;
-  if (numberOfValues === numberOfLayers) {
-    return value;
-  }
-
-  return List.of(
-    (numberOfLayers < numberOfValues
-      ? value.values
-      : Array(Math.ceil(numberOfLayers / numberOfValues))
-          .fill(value.values)
-          .flat()
-    ).slice(0, numberOfLayers),
-    ", ",
-  );
-}
-
 const slash = delimited(option(Token.parseWhitespace), Token.parseDelim("/"));
 
 const parsePosAndSize: CSSParser<[Position, Option<BgSize>]> = pair(
@@ -169,11 +130,11 @@ export default Shorthand.of(
     for (const layer of layers) {
       const [image, pos, size, repeat, origin, clip, composite, mode] = layer;
       images.push(image ?? MaskReference.initialItem);
-      positions.push(pos ?? posInitialItem);
+      positions.push(pos ?? MaskPosition.initialItem);
       sizes.push(size ?? BgSize.initialItem);
       repeats.push(repeat ?? RepeatStyle.initialItem);
-      origins.push(origin ?? originInitialItem);
-      clips.push(clip ?? clipInitialItem);
+      origins.push(origin ?? MaskOrigin.initialItem);
+      clips.push(clip ?? MaskClip.initialItem);
       composites.push(composite ?? CompositingOperator.initialItem);
       modes.push(mode ?? MaskingMode.initialItem);
     }
diff --git a/packages/alfa-style/src/tsconfig.json b/packages/alfa-style/src/tsconfig.json
index 80e803580f..91bba34172 100644
--- a/packages/alfa-style/src/tsconfig.json
+++ b/packages/alfa-style/src/tsconfig.json
@@ -39,6 +39,7 @@
     "./predicate/is-block-container.ts",
     "./predicate/is-flex-container.ts",
     "./predicate/is-grid-container.ts",
+    "./property/helpers/match-layers.ts",
     "./property/background.ts",
     "./property/background-attachment.ts",
     "./property/background-clip.ts",
diff --git a/packages/alfa-style/test/common.ts b/packages/alfa-style/test/common.ts
index e4c9283347..60a8a43413 100755
--- a/packages/alfa-style/test/common.ts
+++ b/packages/alfa-style/test/common.ts
@@ -32,6 +32,17 @@ export function specified<N extends Longhands.Name>(
   return Style.from(element, device, context).specified(name).toJSON();
 }
 
+/**
+ * @internal
+ */
+export function computed<N extends Longhands.Name>(
+  element: Element,
+  name: N,
+  context: Context = Context.empty(),
+): Value.JSON<Style.Specified<N>> {
+  return Style.from(element, device, context).computed(name).toJSON();
+}
+
 /**
  * @internal
  */
diff --git a/packages/alfa-style/test/property/mask-clip.spec.tsx b/packages/alfa-style/test/property/mask-clip.spec.tsx
index 31ece611f7..54f3aae722 100644
--- a/packages/alfa-style/test/property/mask-clip.spec.tsx
+++ b/packages/alfa-style/test/property/mask-clip.spec.tsx
@@ -1,18 +1,12 @@
 import { test } from "@siteimprove/alfa-test";
 import { h } from "@siteimprove/alfa-dom";
 
-import { Device } from "@siteimprove/alfa-device";
-
-import { Style } from "../../dist/index.js";
-
-const device = Device.standard();
+import { computed } from "../common.js";
 
 test("initial value is border-box", (t) => {
   const element = <div></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-clip").toJSON(), {
+  t.deepEqual(computed(element, "mask-clip"), {
     value: {
       type: "list",
       separator: " ",
@@ -39,9 +33,7 @@ test("#computed parses single keywords", (t) => {
   ] as const) {
     const element = <div style={{ maskClip: kw }}></div>;
 
-    const style = Style.from(element, device);
-
-    t.deepEqual(style.computed("mask-clip").toJSON(), {
+    t.deepEqual(computed(element, "mask-clip"), {
       value: {
         type: "list",
         separator: ", ",
@@ -58,14 +50,16 @@ test("#computed parses single keywords", (t) => {
 });
 
 test("#computed parses multiple layers", (t) => {
-  const element = <div style={{
-    maskImage: "url(foo.svg), url(bar.svg)",
-    maskClip: "padding-box, no-clip"
-  }}></div>;
-
-  const style = Style.from(element, device);
+  const element = (
+    <div
+      style={{
+        maskImage: "url(foo.svg), url(bar.svg)",
+        maskClip: "padding-box, no-clip",
+      }}
+    ></div>
+  );
 
-  t.deepEqual(style.computed("mask-clip").toJSON(), {
+  t.deepEqual(computed(element, "mask-clip"), {
     value: {
       type: "list",
       separator: ", ",
@@ -82,18 +76,19 @@ test("#computed parses multiple layers", (t) => {
     },
     source: h.declaration("mask-clip", "padding-box, no-clip").toJSON(),
   });
-
 });
 
 test("#computed discards excess values when there are more values than layers", (t) => {
   const element = (
-    <div style={{
-      maskImage: "url(foo.svg), url(bar.svg)",
-      maskClip: "view-box, fill-box, border-box"
-    }}></div>
+    <div
+      style={{
+        maskImage: "url(foo.svg), url(bar.svg)",
+        maskClip: "view-box, fill-box, border-box",
+      }}
+    ></div>
   );
-  const style = Style.from(element, device);
-  t.deepEqual(style.computed("mask-clip").toJSON(), {
+
+  t.deepEqual(computed(element, "mask-clip"), {
     value: {
       type: "list",
       separator: ", ",
@@ -116,13 +111,15 @@ test("#computed discards excess values when there are more values than layers",
 
 test("#computed repeats values when there are more layers than values", (t) => {
   const element = (
-    <div style={{
-      maskImage: "url(foo.svg), url(bar.svg), url(baz.svg)",
-      maskClip: "view-box, fill-box"
-    }}></div>
+    <div
+      style={{
+        maskImage: "url(foo.svg), url(bar.svg), url(baz.svg)",
+        maskClip: "view-box, fill-box",
+      }}
+    ></div>
   );
-  const style = Style.from(element, device);
-  t.deepEqual(style.computed("mask-clip").toJSON(), {
+
+  t.deepEqual(computed(element, "mask-clip"), {
     value: {
       type: "list",
       separator: ", ",
@@ -141,8 +138,6 @@ test("#computed repeats values when there are more layers than values", (t) => {
         },
       ],
     },
-    source: h
-      .declaration("mask-clip", "view-box, fill-box")
-      .toJSON(),
+    source: h.declaration("mask-clip", "view-box, fill-box").toJSON(),
   });
 });
diff --git a/packages/alfa-style/test/property/mask-composite.spec.tsx b/packages/alfa-style/test/property/mask-composite.spec.tsx
index 0a931218cb..7080334298 100644
--- a/packages/alfa-style/test/property/mask-composite.spec.tsx
+++ b/packages/alfa-style/test/property/mask-composite.spec.tsx
@@ -1,18 +1,12 @@
 import { test } from "@siteimprove/alfa-test";
 import { h } from "@siteimprove/alfa-dom";
 
-import { Device } from "@siteimprove/alfa-device";
-
-import { Style } from "../../dist/index.js";
-
-const device = Device.standard();
+import { computed } from "../common.js";
 
 test("initial value is add", (t) => {
   const element = <div></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-composite").toJSON(), {
+  t.deepEqual(computed(element, "mask-composite"), {
     value: {
       type: "list",
       separator: ", ",
@@ -31,9 +25,7 @@ test("#computed parses single keywords", (t) => {
   for (const kw of ["add", "subtract", "intersect", "exclude"] as const) {
     const element = <div style={{ maskComposite: kw }}></div>;
 
-    const style = Style.from(element, device);
-
-    t.deepEqual(style.computed("mask-composite").toJSON(), {
+    t.deepEqual(computed(element, "mask-composite"), {
       value: {
         type: "list",
         separator: ", ",
@@ -59,9 +51,7 @@ test("#computed parses multiple layers", (t) => {
     ></div>
   );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-composite").toJSON(), {
+  t.deepEqual(computed(element, "mask-composite"), {
     value: {
       type: "list",
       separator: ", ",
@@ -90,9 +80,7 @@ test("#computed discards excess values when there are more values than layers",
     ></div>
   );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-composite").toJSON(), {
+  t.deepEqual(computed(element, "mask-composite"), {
     value: {
       type: "list",
       separator: ", ",
@@ -121,9 +109,7 @@ test("#computed repeats values when there are more layers than values", (t) => {
     ></div>
   );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-composite").toJSON(), {
+  t.deepEqual(computed(element, "mask-composite"), {
     value: {
       type: "list",
       separator: ", ",
diff --git a/packages/alfa-style/test/property/mask-image.spec.tsx b/packages/alfa-style/test/property/mask-image.spec.tsx
index 0ee3a1656e..a50040eb72 100644
--- a/packages/alfa-style/test/property/mask-image.spec.tsx
+++ b/packages/alfa-style/test/property/mask-image.spec.tsx
@@ -1,18 +1,12 @@
 import { test } from "@siteimprove/alfa-test";
 import { h } from "@siteimprove/alfa-dom";
 
-import { Device } from "@siteimprove/alfa-device";
-
-import { Style } from "../../dist/index.js";
-
-const device = Device.standard();
+import { computed } from "../common.js";
 
 test("initial value is none", (t) => {
   const element = <div></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-image").toJSON(), {
+  t.deepEqual(computed(element, "mask-image"), {
     value: {
       type: "list",
       separator: ", ",
@@ -30,9 +24,7 @@ test("initial value is none", (t) => {
 test("#computed parses url value", (t) => {
   const element = <div style={{ maskImage: "url(masks.svg#mask1)" }}></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-image").toJSON(), {
+  t.deepEqual(computed(element, "mask-image"), {
     value: {
       type: "list",
       separator: ", ",
@@ -54,8 +46,8 @@ test("#computed parses linear-gradient value", (t) => {
   const element = (
     <div style={{ maskImage: "linear-gradient(red, blue)" }}></div>
   );
-  const style = Style.from(element, device);
-  t.deepEqual(style.computed("mask-image").toJSON(), {
+
+  t.deepEqual(computed(element, "mask-image"), {
     value: {
       type: "list",
       separator: ", ",
@@ -72,18 +64,48 @@ test("#computed parses linear-gradient value", (t) => {
             items: [
               {
                 color: {
-                  color: "red",
-                  format: "named",
                   type: "color",
+                  format: "rgb",
+                  alpha: {
+                    type: "percentage",
+                    value: 1,
+                  },
+                  red: {
+                    type: "percentage",
+                    value: 1,
+                  },
+                  green: {
+                    type: "percentage",
+                    value: 0,
+                  },
+                  blue: {
+                    type: "percentage",
+                    value: 0,
+                  },
                 },
                 position: null,
                 type: "stop",
               },
               {
                 color: {
-                  color: "blue",
-                  format: "named",
                   type: "color",
+                  format: "rgb",
+                  alpha: {
+                    type: "percentage",
+                    value: 1,
+                  },
+                  red: {
+                    type: "percentage",
+                    value: 0,
+                  },
+                  green: {
+                    type: "percentage",
+                    value: 0,
+                  },
+                  blue: {
+                    type: "percentage",
+                    value: 1,
+                  },
                 },
                 position: null,
                 type: "stop",
@@ -97,3 +119,33 @@ test("#computed parses linear-gradient value", (t) => {
     source: h.declaration("mask-image", "linear-gradient(red, blue)").toJSON(),
   });
 });
+
+test("#computed parses multiple layers", (t) => {
+  const element = (
+    <div style={{ maskImage: "url(foo.svg), url(bar.svg)" }}></div>
+  );
+
+  t.deepEqual(computed(element, "mask-image"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "image",
+          image: {
+            type: "url",
+            url: "foo.svg",
+          },
+        },
+        {
+          type: "image",
+          image: {
+            type: "url",
+            url: "bar.svg",
+          },
+        },
+      ],
+    },
+    source: h.declaration("mask-image", "url(foo.svg), url(bar.svg)").toJSON(),
+  });
+});
diff --git a/packages/alfa-style/test/property/mask-mode.spec.tsx b/packages/alfa-style/test/property/mask-mode.spec.tsx
index 928ce668fc..59e7d916a8 100644
--- a/packages/alfa-style/test/property/mask-mode.spec.tsx
+++ b/packages/alfa-style/test/property/mask-mode.spec.tsx
@@ -1,18 +1,12 @@
 import { test } from "@siteimprove/alfa-test";
 import { h } from "@siteimprove/alfa-dom";
 
-import { Device } from "@siteimprove/alfa-device";
-
-import { Style } from "../../dist/index.js";
-
-const device = Device.standard();
+import { computed } from "../common.js";
 
 test("initial value is match-source", (t) => {
   const element = <div></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-mode").toJSON(), {
+  t.deepEqual(computed(element, "mask-mode"), {
     value: {
       type: "list",
       separator: ", ",
@@ -31,9 +25,7 @@ test("#computed parses single keywords", (t) => {
   for (const kw of ["alpha", "luminance", "match-source"] as const) {
     const element = <div style={{ maskMode: kw }}></div>;
 
-    const style = Style.from(element, device);
-
-    t.deepEqual(style.computed("mask-mode").toJSON(), {
+    t.deepEqual(computed(element, "mask-mode"), {
       value: {
         type: "list",
         separator: ", ",
@@ -58,8 +50,8 @@ test("#computed parses multiple layers", (t) => {
       }}
     ></div>
   );
-  const style = Style.from(element, device);
-  t.deepEqual(style.computed("mask-mode").toJSON(), {
+
+  t.deepEqual(computed(element, "mask-mode"), {
     value: {
       type: "list",
       separator: ", ",
@@ -88,9 +80,7 @@ test("#computed discards excess values when there are more values than layers",
     ></div>
   );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-mode").toJSON(), {
+  t.deepEqual(computed(element, "mask-mode"), {
     value: {
       type: "list",
       separator: ", ",
@@ -121,9 +111,7 @@ test("#computed repeats values when there are more layers than values", (t) => {
     ></div>
   );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-mode").toJSON(), {
+  t.deepEqual(computed(element, "mask-mode"), {
     value: {
       type: "list",
       separator: ", ",
diff --git a/packages/alfa-style/test/property/mask-origin.spec.tsx b/packages/alfa-style/test/property/mask-origin.spec.tsx
index 0ff5bf7437..da10230917 100644
--- a/packages/alfa-style/test/property/mask-origin.spec.tsx
+++ b/packages/alfa-style/test/property/mask-origin.spec.tsx
@@ -1,18 +1,12 @@
 import { test } from "@siteimprove/alfa-test";
 import { h } from "@siteimprove/alfa-dom";
 
-import { Device } from "@siteimprove/alfa-device";
-
-import { Style } from "../../dist/index.js";
-
-const device = Device.standard();
+import { computed } from "../common.js";
 
 test("initial value is border-box", (t) => {
   const element = <div></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-origin").toJSON(), {
+  t.deepEqual(computed(element, "mask-origin"), {
     value: {
       type: "list",
       separator: ", ",
@@ -38,9 +32,7 @@ test("#computed parses single keywords", (t) => {
   ] as const) {
     const element = <div style={{ maskOrigin: kw }}></div>;
 
-    const style = Style.from(element, device);
-
-    t.deepEqual(style.computed("mask-origin").toJSON(), {
+    t.deepEqual(computed(element, "mask-origin"), {
       value: {
         type: "list",
         separator: ", ",
@@ -66,9 +58,7 @@ test("#computed parses multiple layers", (t) => {
     ></div>
   );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-origin").toJSON(), {
+  t.deepEqual(computed(element, "mask-origin"), {
     value: {
       type: "list",
       separator: ", ",
@@ -97,9 +87,7 @@ test("#computed discards excess values when there are more values than layers",
     ></div>
   );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-origin").toJSON(), {
+  t.deepEqual(computed(element, "mask-origin"), {
     value: {
       type: "list",
       separator: ", ",
@@ -130,9 +118,7 @@ test("#computed repeats values when there are more layers than values", (t) => {
     ></div>
   );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-origin").toJSON(), {
+  t.deepEqual(computed(element, "mask-origin"), {
     value: {
       type: "list",
       separator: ", ",
diff --git a/packages/alfa-style/test/property/mask-position.spec.tsx b/packages/alfa-style/test/property/mask-position.spec.tsx
index c24726391e..c35669ebce 100644
--- a/packages/alfa-style/test/property/mask-position.spec.tsx
+++ b/packages/alfa-style/test/property/mask-position.spec.tsx
@@ -1,18 +1,12 @@
 import { test } from "@siteimprove/alfa-test";
 import { h } from "@siteimprove/alfa-dom";
 
-import { Device } from "@siteimprove/alfa-device";
-
-import { Style } from "../../dist/index.js";
-
-const device = Device.standard();
+import { computed } from "../common.js";
 
 test("initial value is 0% 0%", (t) => {
   const element = <div></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-position").toJSON(), {
+  t.deepEqual(computed(element, "mask-position"), {
     value: {
       type: "list",
       separator: ", ",
@@ -54,9 +48,7 @@ test("#computed parses single keywords", (t) => {
   for (const kw of ["top", "bottom"] as const) {
     const element = <div style={{ maskPosition: kw }}></div>;
 
-    const style = Style.from(element, device);
-
-    t.deepEqual(style.computed("mask-position").toJSON(), {
+    t.deepEqual(computed(element, "mask-position"), {
       value: {
         type: "list",
         separator: ", ",
@@ -85,9 +77,7 @@ test("#computed parses single keywords", (t) => {
   for (const kw of ["left", "right"] as const) {
     const element = <div style={{ maskPosition: kw }}></div>;
 
-    const style = Style.from(element, device);
-
-    t.deepEqual(style.computed("mask-position").toJSON(), {
+    t.deepEqual(computed(element, "mask-position"), {
       value: {
         type: "list",
         separator: ", ",
@@ -115,9 +105,7 @@ test("#computed parses single keywords", (t) => {
 
   const element = <div style={{ maskPosition: "center" }}></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-position").toJSON(), {
+  t.deepEqual(computed(element, "mask-position"), {
     value: {
       type: "list",
       separator: ", ",
@@ -142,9 +130,7 @@ test("#computed parses single keywords", (t) => {
 test("#computed parses lengths and percentages", (t) => {
   const element = <div style={{ maskPosition: "10% 3em" }}></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-position").toJSON(), {
+  t.deepEqual(computed(element, "mask-position"), {
     value: {
       type: "list",
       separator: ", ",
@@ -191,9 +177,7 @@ test("#computed parses multiple layers", (t) => {
     ></div>
   );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-position").toJSON(), {
+  t.deepEqual(computed(element, "mask-position"), {
     value: {
       type: "list",
       separator: ", ",
diff --git a/packages/alfa-style/test/property/mask-repeat.spec.tsx b/packages/alfa-style/test/property/mask-repeat.spec.tsx
index cf5862c926..34816be52e 100644
--- a/packages/alfa-style/test/property/mask-repeat.spec.tsx
+++ b/packages/alfa-style/test/property/mask-repeat.spec.tsx
@@ -1,18 +1,12 @@
 import { test } from "@siteimprove/alfa-test";
 import { h } from "@siteimprove/alfa-dom";
 
-import { Device } from "@siteimprove/alfa-device";
-
-import { Style } from "../../dist/index.js";
-
-const device = Device.standard();
+import { computed } from "../common.js";
 
 test("initial value is repeat", (t) => {
   const element = <div></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-repeat").toJSON(), {
+  t.deepEqual(computed(element, "mask-repeat"), {
     value: {
       type: "list",
       separator: ", ",
@@ -37,9 +31,7 @@ test("#computed parses single keywords", (t) => {
   for (const kw of ["repeat-x", "repeat-y"] as const) {
     const element = <div style={{ maskRepeat: kw }}></div>;
 
-    const style = Style.from(element, device);
-
-    t.deepEqual(style.computed("mask-repeat").toJSON(), {
+    t.deepEqual(computed(element, "mask-repeat"), {
       value: {
         type: "list",
         separator: ", ",
@@ -57,9 +49,7 @@ test("#computed parses single keywords", (t) => {
   for (const kw of ["repeat", "space", "round", "no-repeat"] as const) {
     const element = <div style={{ maskRepeat: kw }}></div>;
 
-    const style = Style.from(element, device);
-
-    t.deepEqual(style.computed("mask-repeat").toJSON(), {
+    t.deepEqual(computed(element, "mask-repeat"), {
       value: {
         type: "list",
         separator: ", ",
@@ -70,8 +60,8 @@ test("#computed parses single keywords", (t) => {
             values: [
               {
                 type: "keyword",
-                value: kw
-              }
+                value: kw,
+              },
             ],
           },
         ],
@@ -83,8 +73,7 @@ test("#computed parses single keywords", (t) => {
 
 test("#computed parses at most two space separated values", (t) => {
   const element1 = <div style={{ maskRepeat: "repeat space" }}></div>;
-  const style1 = Style.from(element1, device);
-  t.deepEqual(style1.computed("mask-repeat").toJSON(), {
+  t.deepEqual(computed(element1, "mask-repeat"), {
     value: {
       type: "list",
       separator: ", ",
@@ -109,8 +98,7 @@ test("#computed parses at most two space separated values", (t) => {
   });
 
   const element2 = <div style={{ maskRepeat: "repeat space round" }}></div>;
-  const style2 = Style.from(element2, device);
-  t.deepEqual(style2.computed("mask-repeat").toJSON(), {
+  t.deepEqual(computed(element2, "mask-repeat"), {
     value: {
       type: "list",
       separator: ", ",
@@ -132,14 +120,16 @@ test("#computed parses at most two space separated values", (t) => {
 });
 
 test("#computed parses mutiple layers", (t) => {
-  const element = <div style={{
-    maskImage: "url(foo.svg), url(bar.svg)",
-    maskRepeat: "round repeat, space"
-  }}></div>;
-
-  const style = Style.from(element, device);
+  const element = (
+    <div
+      style={{
+        maskImage: "url(foo.svg), url(bar.svg)",
+        maskRepeat: "round repeat, space",
+      }}
+    ></div>
+  );
 
-  t.deepEqual(style.computed("mask-repeat").toJSON(), {
+  t.deepEqual(computed(element, "mask-repeat"), {
     value: {
       type: "list",
       separator: ", ",
@@ -150,12 +140,12 @@ test("#computed parses mutiple layers", (t) => {
           values: [
             {
               type: "keyword",
-              value: "round"
+              value: "round",
             },
             {
               type: "keyword",
-              value: "repeat"
-            }
+              value: "repeat",
+            },
           ],
         },
         {
@@ -163,12 +153,11 @@ test("#computed parses mutiple layers", (t) => {
           separator: " ",
           values: [
             {
-
               type: "keyword",
-              value: "space"
-            }
-          ]
-        }
+              value: "space",
+            },
+          ],
+        },
       ],
     },
     source: h.declaration("mask-repeat", "round repeat, space").toJSON(),
@@ -176,14 +165,16 @@ test("#computed parses mutiple layers", (t) => {
 });
 
 test("#computed discards excess values when there are more values than layers", (t) => {
-  const element = <div style={{
-    maskImage: "url(foo.svg)",
-    maskRepeat: "round repeat, space"
-  }}></div>;
+  const element = (
+    <div
+      style={{
+        maskImage: "url(foo.svg)",
+        maskRepeat: "round repeat, space",
+      }}
+    ></div>
+  );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-repeat").toJSON(), {
+  t.deepEqual(computed(element, "mask-repeat"), {
     value: {
       type: "list",
       separator: ", ",
@@ -194,12 +185,12 @@ test("#computed discards excess values when there are more values than layers",
           values: [
             {
               type: "keyword",
-              value: "round"
+              value: "round",
             },
             {
               type: "keyword",
-              value: "repeat"
-            }
+              value: "repeat",
+            },
           ],
         },
       ],
@@ -209,14 +200,16 @@ test("#computed discards excess values when there are more values than layers",
 });
 
 test("#computed repeats values when there are more layers than values", (t) => {
-  const element = <div style={{
-    maskImage: "url(foo.svg), url(bar.svg), url(baz.svg)",
-    maskRepeat: "round repeat, space"
-  }}></div>;
+  const element = (
+    <div
+      style={{
+        maskImage: "url(foo.svg), url(bar.svg), url(baz.svg)",
+        maskRepeat: "round repeat, space",
+      }}
+    ></div>
+  );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-repeat").toJSON(), {
+  t.deepEqual(computed(element, "mask-repeat"), {
     value: {
       type: "list",
       separator: ", ",
@@ -227,12 +220,12 @@ test("#computed repeats values when there are more layers than values", (t) => {
           values: [
             {
               type: "keyword",
-              value: "round"
+              value: "round",
             },
             {
               type: "keyword",
-              value: "repeat"
-            }
+              value: "repeat",
+            },
           ],
         },
         {
@@ -240,11 +233,10 @@ test("#computed repeats values when there are more layers than values", (t) => {
           separator: " ",
           values: [
             {
-
               type: "keyword",
-              value: "space"
-            }
-          ]
+              value: "space",
+            },
+          ],
         },
         {
           type: "list",
@@ -252,12 +244,12 @@ test("#computed repeats values when there are more layers than values", (t) => {
           values: [
             {
               type: "keyword",
-              value: "round"
+              value: "round",
             },
             {
               type: "keyword",
-              value: "repeat"
-            }
+              value: "repeat",
+            },
           ],
         },
       ],
diff --git a/packages/alfa-style/test/property/mask-size.spec.tsx b/packages/alfa-style/test/property/mask-size.spec.tsx
index b72b9a5316..08d3ead997 100644
--- a/packages/alfa-style/test/property/mask-size.spec.tsx
+++ b/packages/alfa-style/test/property/mask-size.spec.tsx
@@ -1,11 +1,7 @@
 import { test } from "@siteimprove/alfa-test";
 import { h } from "@siteimprove/alfa-dom";
 
-import { Device } from "@siteimprove/alfa-device";
-
-import { Style } from "../../dist/index.js";
-
-const device = Device.standard();
+import { computed } from "../common.js";
 
 test("initial value is auto", (t) => {
   const element = (
@@ -16,9 +12,7 @@ test("initial value is auto", (t) => {
     ></div>
   );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-size").toJSON(), {
+  t.deepEqual(computed(element, "mask-size"), {
     value: {
       type: "list",
       separator: ", ",
@@ -53,9 +47,7 @@ test("#computed parses single keywords", (t) => {
   for (const kw of ["cover", "contain"] as const) {
     const element = <div style={{ maskSize: kw }}></div>;
 
-    const style = Style.from(element, device);
-
-    t.deepEqual(style.computed("mask-size").toJSON(), {
+    t.deepEqual(computed(element, "mask-size"), {
       value: {
         type: "list",
         separator: ", ",
@@ -74,9 +66,7 @@ test("#computed parses single keywords", (t) => {
 test("#computed parses percentage width", (t) => {
   const element = <div style={{ maskSize: "50%" }}></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-size").toJSON(), {
+  t.deepEqual(computed(element, "mask-size"), {
     value: {
       type: "list",
       separator: ", ",
@@ -100,9 +90,7 @@ test("#computed parses percentage width", (t) => {
 test("#computed resolves em width", (t) => {
   const element = <div style={{ maskSize: "3em" }}></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-size").toJSON(), {
+  t.deepEqual(computed(element, "mask-size"), {
     value: {
       type: "list",
       separator: ", ",
@@ -127,9 +115,7 @@ test("#computed resolves em width", (t) => {
 test("#computed parses pixel width", (t) => {
   const element = <div style={{ maskSize: "12px" }}></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-size").toJSON(), {
+  t.deepEqual(computed(element, "mask-size"), {
     value: {
       type: "list",
       separator: ", ",
@@ -154,9 +140,7 @@ test("#computed parses pixel width", (t) => {
 test("#computed parses width and height", (t) => {
   const element = <div style={{ maskSize: "3em 25%" }}></div>;
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-size").toJSON(), {
+  t.deepEqual(computed(element, "mask-size"), {
     value: {
       type: "list",
       separator: ", ",
@@ -192,9 +176,7 @@ test("#computed parses multiple layers", (t) => {
     ></div>
   );
 
-  const style = Style.from(element, device);
-
-  t.deepEqual(style.computed("mask-size").toJSON(), {
+  t.deepEqual(computed(element, "mask-size"), {
     value: {
       type: "list",
       separator: ", ",
diff --git a/packages/alfa-style/test/property/mask.spec.tsx b/packages/alfa-style/test/property/mask.spec.tsx
new file mode 100644
index 0000000000..73e87fbfb2
--- /dev/null
+++ b/packages/alfa-style/test/property/mask.spec.tsx
@@ -0,0 +1,343 @@
+import { test } from "@siteimprove/alfa-test";
+import { h } from "@siteimprove/alfa-dom";
+
+import { computed } from "../common.js";
+
+test("longhands resolve correctly from shorthand", (t) => {
+  const mask =
+    "url(foo.svg) 50% 0% / 12px repeat-x view-box padding-box subtract luminance";
+  const decl = h.declaration("mask", mask);
+  const element = <div style={{ mask }}></div>;
+
+  t.deepEqual(computed(element, "mask-image"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "image",
+          image: {
+            type: "url",
+            url: "foo.svg",
+          },
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+
+  t.deepEqual(computed(element, "mask-position"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "position",
+          horizontal: {
+            type: "side",
+            offset: {
+              type: "percentage",
+              value: 0.5,
+            },
+            side: {
+              type: "keyword",
+              value: "left",
+            },
+          },
+          vertical: {
+            type: "side",
+            offset: {
+              type: "percentage",
+              value: 0,
+            },
+            side: {
+              type: "keyword",
+              value: "top",
+            },
+          },
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+
+  t.deepEqual(computed(element, "mask-size"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "list",
+          separator: " ",
+          values: [
+            {
+              type: "length",
+              unit: "px",
+              value: 12,
+            },
+          ],
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+
+  t.deepEqual(computed(element, "mask-repeat"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "keyword",
+          value: "repeat-x",
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+
+  t.deepEqual(computed(element, "mask-origin"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "keyword",
+          value: "view-box",
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+
+  t.deepEqual(computed(element, "mask-clip"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "keyword",
+          value: "padding-box",
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+
+  t.deepEqual(computed(element, "mask-composite"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "keyword",
+          value: "subtract",
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+
+  t.deepEqual(computed(element, "mask-mode"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "keyword",
+          value: "luminance",
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+});
+
+test("if one `<coord-box>` value and the `no-clip` keyword are present then `<coord-box>` sets `mask-origin` and `no-clip` sets `mask-clip`", (t) => {
+  const mask = "url(foo.svg) view-box no-clip";
+  const decl = h.declaration("mask", mask);
+  const element = <div style={{ mask }}></div>;
+
+  t.deepEqual(computed(element, "mask-origin"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "keyword",
+          value: "view-box",
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+
+  t.deepEqual(computed(element, "mask-clip"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "keyword",
+          value: "no-clip",
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+});
+
+test("if one `<coord-box>` value and no `no-clip` keyword are present then `<coord-box>` sets both `mask-origin` and `mask-clip`", (t) => {
+  const mask = "url(foo.svg) view-box";
+  const decl = h.declaration("mask", mask);
+  const element = <div style={{ mask }}></div>;
+
+  t.deepEqual(computed(element, "mask-origin"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "keyword",
+          value: "view-box",
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+
+  t.deepEqual(computed(element, "mask-clip"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "keyword",
+          value: "view-box",
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+});
+
+test("longhands resolves correctly from shorthand with layers", (t) => {
+  const mask = "url(foo.svg) 50% 0% / 12px, url(bar.svg) 0% 50%";
+  const decl = h.declaration("mask", mask);
+  const element = <div style={{ mask }}></div>;
+
+  t.deepEqual(computed(element, "mask-image"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "image",
+          image: {
+            type: "url",
+            url: "foo.svg",
+          },
+        },
+        {
+          type: "image",
+          image: {
+            type: "url",
+            url: "bar.svg",
+          },
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+
+  t.deepEqual(computed(element, "mask-position"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "position",
+          horizontal: {
+            type: "side",
+            offset: {
+              type: "percentage",
+              value: 0.5,
+            },
+            side: {
+              type: "keyword",
+              value: "left",
+            },
+          },
+          vertical: {
+            type: "side",
+            offset: {
+              type: "percentage",
+              value: 0,
+            },
+            side: {
+              type: "keyword",
+              value: "top",
+            },
+          },
+        },
+        {
+          type: "position",
+          horizontal: {
+            type: "side",
+            offset: {
+              type: "percentage",
+              value: 0,
+            },
+            side: {
+              type: "keyword",
+              value: "left",
+            },
+          },
+          vertical: {
+            type: "side",
+            offset: {
+              type: "percentage",
+              value: 0.5,
+            },
+            side: {
+              type: "keyword",
+              value: "top",
+            },
+          },
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+
+  t.deepEqual(computed(element, "mask-size"), {
+    value: {
+      type: "list",
+      separator: ", ",
+      values: [
+        {
+          type: "list",
+          separator: " ",
+          values: [
+            {
+              type: "length",
+              unit: "px",
+              value: 12,
+            },
+          ],
+        },
+        {
+          type: "list",
+          separator: " ",
+          values: [
+            {
+              type: "keyword",
+              value: "auto",
+            },
+          ],
+        },
+      ],
+    },
+    source: decl.toJSON(),
+  });
+});
diff --git a/packages/alfa-style/test/tsconfig.json b/packages/alfa-style/test/tsconfig.json
index a4415680a6..40bb05d75e 100644
--- a/packages/alfa-style/test/tsconfig.json
+++ b/packages/alfa-style/test/tsconfig.json
@@ -62,6 +62,7 @@
     "./property/mask-position.spec.tsx",
     "./property/mask-repeat.spec.tsx",
     "./property/mask-size.spec.tsx",
+    "./property/mask.spec.tsx",
     "./property/mix-blend-mode.spec.tsx",
     "./property/opacity.spec.tsx",
     "./property/outline.spec.tsx",