diff --git a/css/components/headernav.css b/css/components/headernav.css
index f3dd3ec..23ac5ec 100644
--- a/css/components/headernav.css
+++ b/css/components/headernav.css
@@ -6,10 +6,9 @@
   --headernav-section-padding: var(--space3);
   --headernav-item-margin: var(--space3);
 
-  position: relative;
-
   ul {
-    all: unset;
+    margin: unset;
+    padding: unset;
     list-style-type: none;
 
     li {
@@ -60,9 +59,28 @@
   }
 
   /* Panels: */
+
+  [popover] {
+    position-area: block-end span-inline-end;
+    position-try-fallbacks: flip-inline;
+    inset: unset;
+    margin: 0;
+    transition-property: opacity, display, overlay;
+    transition-duration: .3s;
+    transition-behavior: allow-discrete;
+    border: unset;
+    opacity: 0;
+  
+    &:popover-open {
+      opacity: 1;
+  
+      @starting-style {
+        opacity: 0;
+      }
+    }
+  }
+
   > ul > li > ul {
-    display: none;
-    position: absolute;
     padding: var(--headernav-section-padding);
     column-gap: calc(var(--headernav-section-padding) * 2);
     column-rule: 1px solid oklch(88% 0 0deg);
@@ -126,48 +144,13 @@
   > ul > li li {
     margin-block-end: var(--headernav-item-margin);
   }
+}
 
-  /**** Panel Toggle ****/
-
-  /* If c-headernav JS not supported: */
-
-  &.c-headernav-no-js {
-    > ul > li:hover > ul {
-      @media (--min-width2) {
-        display: block;
-        z-index: 10;
-      }
-    }
-
-    /* If :focus-within not supported. Only primary links accessible via tabkey: */
-
-    > ul > li a:focus {
-      @media (--min-width2) {
-        display: block;
-        z-index: 10;
-      }
-    }
-
-    /* If :focus-within supported. All links accessbile via focus. No toggle state on primary links, so user must tab through every secondary link in each section to reach the next section: */
-
-    > ul > li:focus-within > ul {
-      @media (--min-width2) {
-        display: block;
-        z-index: 10;
-      }
-    }
-  }
-
-  /* If c-headernav JS supported: */
-
-  &.c-headernav-js {
-    /* Primary links disabled via JS and used as a toggle for each section. Hover and focus states handled by JS and set via 'open' class: */
+.c-header ~ * {
+  filter: blur(0) contrast(1) grayscale(0);
+  transition: filter 0.5s;
+}
 
-    li.open ul {
-      @media (--min-width2) {
-        display: block;
-        z-index: 10;
-      }
-    }
-  }
+.c-header:has(:popover-open) ~ * {
+  filter: blur(0.05rem) contrast(90%) grayscale(10%);
 }
diff --git a/elements/3-components/headernav.hbs b/elements/3-components/headernav.hbs
index c8a8914..ab60862 100644
--- a/elements/3-components/headernav.hbs
+++ b/elements/3-components/headernav.hbs
@@ -1,5 +1,5 @@
 {{!-- Read notes in component config --}}
-<nav aria-label="global" class="c-headernav c-headernav-no-js c-mobilenav">
+<nav aria-label="global" class="c-headernav c-mobilenav js-headernav">
   <ul>
     {{> @menu-items}}
   </ul>
diff --git a/js/headernav.js b/js/headernav.js
index f456dd8..a039a78 100644
--- a/js/headernav.js
+++ b/js/headernav.js
@@ -1,60 +1,61 @@
 // Headernav Component:
 
 const headerNavMediaQuery = window.matchMedia('(min-width: 760px)')
-
-function clickLink (event) {
-  if (this.parentElement.classList.contains('open') === false) {
-    this.parentElement.classList.add('open')
-    this.setAttribute('aria-expanded', 'true')
-  } else {
-    this.parentElement.classList.remove('open')
-    this.setAttribute('aria-expanded', 'false')
-  }
-  event.preventDefault()
-}
+const menu = document.querySelector('.js-headernav')
+const subNavs = menu.querySelectorAll(':scope > ul > li:has(> ul)')
+let counter = 1
 
 const headerNavToggles = e => {
   if (document.querySelector('.c-headernav') && e.matches) {
-    const allMenuItems = document.querySelectorAll('.c-headernav > ul > li');
-
-    [].forEach.call(allMenuItems, function (el) {
-      document.querySelector('.c-headernav').classList.remove('c-headernav-no-js')
-      document.querySelector('.c-headernav').classList.add('c-headernav-js')
-      el.querySelector('a').setAttribute('aria-haspopup', 'true')
-      el.querySelector('a').setAttribute('aria-expanded', 'false')
-
-      el.addEventListener('mouseover', function (event) {
-        this.classList.add('open')
-        this.querySelector('a').setAttribute('aria-expanded', 'true')
+    // console.log('page wdith at least 760px')
+
+    // Find all lists items containing a nested list (subnav). Set each subnav sibling link and parent list item as controls for the subnav, then set the popover:
+    for (const subNav of subNavs) {
+      const subNavSiblingLink = subNav.querySelector('a')
+      const subNavPopover = subNav.querySelector('ul')
+      subNavPopover.popover = ''
+
+      // Anchor each sibling link to its popover using unique anchor names:
+      const anchorName = '--anchor' + counter++
+      subNavSiblingLink.style.setProperty('anchor-name', anchorName)
+      subNavPopover.style.setProperty('position-anchor', anchorName)
+
+      // Only a <button> as a popover control has built-in accessiblity bindings with popovers. So for the sibling links, we need to set and toggle their aria expanded state:
+      subNavSiblingLink.setAttribute('aria-expanded', 'false') // initial state
+
+      const expandedState = () => {
+        if (subNavPopover.matches(':popover-open')) {
+          subNavSiblingLink.setAttribute('aria-expanded', 'true')
+        } else {
+          subNavSiblingLink.setAttribute('aria-expanded', 'false')
+        }
+      }
+
+      // Show/hide subnav on mouse pointer over and out. Omit these two event listeners if this effect is not desired:
+      subNav.addEventListener('mouseover', () => {
+        subNavPopover.showPopover()
+        expandedState()
       })
 
-      el.addEventListener('mouseout', function (event) {
-        this.classList.remove('open')
-        this.querySelector('a').setAttribute('aria-expanded', 'false')
+      subNav.addEventListener('mouseout', () => {
+        subNavPopover.hidePopover()
+        expandedState()
       })
 
-      el.querySelector('a').addEventListener('click', clickLink)
-    });
-
-    [].forEach.call(allMenuItems, function (el) {
-      el.querySelector('a').addEventListener('focus', function (event) {
-        [].forEach.call(
-          allMenuItems,
-          function (el) {
-            if (el !== this.parentElement) {
-              el.classList.remove('open')
-              el.querySelector('a').setAttribute('aria-expanded', 'false')
-            }
-          }, this
-        )
+      // Toggle subnav if sibling link is clicked by keyboard return key:
+      subNavSiblingLink.addEventListener('click', () => {
+        subNavPopover.togglePopover()
+        expandedState()
       })
-    })
-  } else {
-    const allMenuItems = document.querySelectorAll('.c-headernav > ul > li');
 
-    [].forEach.call(allMenuItems, function (el) {
-      el.querySelector('a').removeEventListener('click', clickLink)
-    })
+      // Hide subnav when keyboard focus leaves popover control: Adapted from https://danburzo.ro/focus-within
+      subNav.addEventListener('focusout', (e) => {
+        if (!e.currentTarget.contains(e.relatedTarget)) {
+          subNavPopover.hidePopover()
+          expandedState()
+        }
+      })
+    }
   }
 }