Add experimental h1 UA styles removal

files/en-us/mozilla/firefox/experimental_features/index.md

@@ -775,7 +775,7 @@ See ([Firefox bug 1855763](https://bugzil.la/1855763) and [Firefox bug 390936](h
   </tbody>
 </table>
 
-To ensure compatibility with these changes, the [Vendor-prefixed transform properties](#vendor-prefixed-transform-properties) and the [Vendor-prefixed transition properties](#vendor-prefixed-transition-properties) are disabled in the Nightly release.
+To ensure compatibility with these changes, the [Vendor-prefixed transform properties](#vendor-prefixed_transform_properties) and the [Vendor-prefixed transition properties](#vendor-prefixed_transition_properties) are disabled in the Nightly release.
 These changes are described in the following sections.
 
 ### text-wrap: balance & stable values
@@ -819,9 +819,9 @@ The [`text-wrap`](/en-US/docs/Web/CSS/text-wrap) CSS property values `balance` a
   </tbody>
 </table>
 
-#### Vendor-prefixed transform properties
+### Vendor-prefixed transform properties
 
-The `-moz-` prefixed [CSS transform](/en-US/docs/Web/CSS/CSS_transforms) properties have been disabled in the Nightly release via the `layout.css.prefixes.transforms` preference being set to `false` ([Firefox bug 1855763](https://bugzil.la/1855763)).
+The `-moz-` prefixed [CSS transform](/en-US/docs/Web/CSS/CSS_transforms) properties have been disabled in the Nightly release via the `layout.css.prefixes.transforms` preference being set to `false`. ([Firefox bug 1855763](https://bugzil.la/1855763)).
 Specifically, the disabled properties are:
 
 - `-moz-backface-visibility`
@@ -869,9 +869,9 @@ Specifically, the disabled properties are:
   </tbody>
 </table>
 
-#### Vendor-prefixed transition properties
+### Vendor-prefixed transition properties
 
-The `-moz-` prefixed [CSS transitions](/en-US/docs/Web/CSS/CSS_transitions) properties have been disabled in the Nightly release via the `layout.css.prefixes.transitions` preference being set to `false` ([Firefox bug 1855763](https://bugzil.la/1855763)).
+The `-moz-` prefixed [CSS transitions](/en-US/docs/Web/CSS/CSS_transitions) properties have been disabled in the Nightly release via the `layout.css.prefixes.transitions` preference being set to `false`. ([Firefox bug 1855763](https://bugzil.la/1855763)).
 Specifically, the disabled properties are:
 
 - `-moz-transition`
@@ -918,6 +918,50 @@ Specifically, the disabled properties are:
   </tbody>
 </table>
 
+### UA styles for `<h1>` nested in sectioning elements
+
+The `<h1>` heading doesn't decrease in font size now when nested within [sectioning elements](/en-US/docs/Web/HTML/Content_categories#sectioning_content) `<article>`, `<aside>`, `<nav>`, and `<section>`. The UA styles for `<h1>` nested within sectioning elements are no longer relevant since the outline algorithm [has been removed](https://github.com/whatwg/html/pull/7829) from the HTML specification. ([Firefox bug 1883896](https://bugzil.la/1883896)).
+
+> **Note:** The preference for this feature works in reverse: it's set to `false` in the Nightly build, which removes the UA styling for headings nested in sectioning elements. It's set to `true` in all other channels, which retains the existing UA styling for the nested headings.
+
+<table>
+  <thead>
+    <tr>
+      <th>Release channel</th>
+      <th>Version added</th>
+      <th>Enabled by default?</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <th>Nightly</th>
+      <td>125</td>
+      <td>Yes</td>
+    </tr>
+    <tr>
+      <th>Developer Edition</th>
+      <td>125</td>
+      <td>No</td>
+    </tr>
+    <tr>
+      <th>Beta</th>
+      <td>125</td>
+      <td>No</td>
+    </tr>
+    <tr>
+      <th>Release</th>
+      <td>125</td>
+      <td>No</td>
+    </tr>
+    <tr>
+      <th>Preference name</th>
+      <td colspan="2">
+        <code>layout.css.h1-in-section-ua-styles.enabled</code>
+      </td>
+    </tr>
+  </tbody>
+</table>
+
 ## SVG
 
 ### SVGPathSeg APIs

files/en-us/mozilla/firefox/releases/125/index.md

@@ -70,6 +70,12 @@ This article provides information about the changes in Firefox 125 that affect d
 
 These features are newly shipped in Firefox 125 but are disabled by default. To experiment with them, search for the appropriate preference on the `about:config` page and set it to `true`. You can find more such features on the [Experimental features](/en-US/docs/Mozilla/Firefox/Experimental_features) page.
 
+- **UA styles for `<h1>` nested into sectioning elements:** `layout.css.h1-in-section-ua-styles.enabled`.
+
+  The `<h1>` heading doesn't decrease in font size now when nested within [sectioning elements](/en-US/docs/Web/HTML/Content_categories#sectioning_content) `<article>`, `<aside>`, `<nav>`, and `<section>`. The UA styles for `<h1>` nested within sectioning elements are no longer relevant since the outline algorithm [has been removed](https://github.com/whatwg/html/pull/7829) from the HTML specification. ([Firefox bug 1883896](https://bugzil.la/1883896)).
+
+  > **Note:** The preference for this feature works in reverse: it's set to `false` in the Nightly build, which removes the UA styling for headings nested in sectioning elements. It's set to `true` in all other channels, which retains the existing UA styling for the nested headings.
+
 ## Older versions
 
 {{Firefox_for_developers}}