Convert element coordinates to document origin

The spec is supposed to normalise everything to document coordinates
for computing intersections. However element bounding boxes turn out
to be in viewport coordinates, so we need to uncinditionally convert
these before using them.

index.bs

@@ -2436,7 +2436,7 @@ To <dfn>encode a canvas as Base64</dfn> given |canvas| and |format|:
 1. Otherwise, let |type| be "image/png" and let |quality| be [=undefined=].
 
 1. Let |file| be [=a serialization of the bitmap as a file=] for |canvas| with
-   |type| and |quality|. 
+   |type| and |quality|.
 
 1. Let |encoded string| be the [=forgiving-base64 encode=] of |file|.
 
@@ -2519,7 +2519,16 @@ The [=remote end steps=] with <var ignore>session</var> and |command parameters|
        1. If |element|'s [=node document=] is not |document|, return [=error=] with
           [=error code=] [=no such element=].
 
-       1. Let |clip rect| be [=get the bounding box=] for |element|.
+       1. Let |viewport rect| be [=get the origin rectangle=] given
+          "<code>viewport</code>" and "<code>document</code>".
+
+       1. Let |element rect| be [=get the bounding box=] for |element|.
+
+       1. Let |clip rect| be a {{DOMRectReadOnly}} with [=x coordinate=]
+          |element rect|["<code>x</code>"] + |viewport rect|["<code>x</code>"]|,
+          [=y coordinate=] |element rect|["<code>y</code>"] + |viewport
+          rect|["<code>y</code>"], width |element rect|["<code>width</code>"],
+          and height |element rect|["<code>height</code>"].
 
        <dt>|clip| matches the <code>browsingContext.BoxClipRectangle</code> production:
        <dd>
@@ -2530,10 +2539,12 @@ The [=remote end steps=] with <var ignore>session</var> and |command parameters|
           coordinate=].
 
        1. Let |clip rect| be a {{DOMRectReadOnly}} with [=x coordinate=] |clip x|,
-          |clip y|, width |clip|["<code>width</code>"], and height
+          [=y coordinate=] |clip y|, width |clip|["<code>width</code>"], and height
           |clip|["<code>height</code>"].
      </dl>
 
+1. Note: All coordiantes are now measured from the origin of the document.
+
 1. Let |rect| be the [=rectangle intersection=] of |origin rect| and |clip rect|.
 
 1. If |rect|'s [=width dimension=] is 0 or |rect|'s [=height dimension=] is 0,