added ff release notes for :heading pseudo-classes (mdn#40655)

* added ff release notes for :heading pseudo-classes

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

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

* updated the 142 experimental note

---------

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

Author: dletorey

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

@@ -337,6 +337,20 @@ The CSS {{CSSXRef("anchor-size")}} function enables setting anchor-positioned el
 - `layout.css.anchor-positioning.enabled`
   - : Set to `true` to enable.
 
+### `:heading` and `:heading()` pseudo-classes
+
+The {{CSSXRef(":heading")}} pseudo-class allows you to style all [heading elements](/en-US/docs/Web/HTML/Reference/Elements/Heading_Elements) (`<h1>`-`<h6>`) at once, rather than targeting them individually. The {{CSSXRef(":heading_function", ":heading()")}} functional pseudo-class allows you to style heading elements that match the [`<An+B>`](/en-US/docs/Web/CSS/:heading_function#functional_notation) notation. ([Firefox bug 1974386](https://bugzil.la/1974386)).
+
+| Release channel   | Version added | Enabled by default? |
+| ----------------- | ------------- | ------------------- |
+| Nightly           | 142           | No                  |
+| Developer Edition | 142           | No                  |
+| Beta              | 142           | No                  |
+| Release           | 142           | No                  |
+
+- `layout.css.heading-selector.enabled`
+  - : Set to `true` to enable.
+
 ## SVG
 
 **No experimental features in this release cycle.**

files/en-us/mozilla/firefox/releases/142/index.md

@@ -89,6 +89,10 @@ Firefox 142 is the current [Beta version of Firefox](https://www.firefox.com/en-
 
   The CSS {{CSSXRef("anchor-size")}} function enables setting anchor-positioned element's size, position, and margins relative to the dimensions of anchor elements. ([Firefox bug 1972610](https://bugzil.la/1972610)).
 
+- **`:heading`** and **`:heading()`**: `layout.css.anchor-positioning.enabled`
+
+  The CSS {{CSSXRef(":heading")}} pseudo-class allows you to style all [heading elements](/en-US/docs/Web/HTML/Reference/Elements/Heading_Elements) (`<h1>`-`<h6>`) at once rather than targeting them individually. The {{CSSXRef(":heading_function", ":heading()")}} functional pseudo-class allows you to style heading elements that match the [`<An+B>`](/en-US/docs/Web/CSS/:heading_function#functional_notation) notation. ([Firefox bug 1974386](https://bugzil.la/1974386)).
+
 - **`Integrity-Policy` and `Integrity-Policy-Report-Only`** (Nightly): `security.integrity_policy.enabled`
 
   The {{httpheader("Integrity-Policy")}} and {{httpheader("Integrity-Policy-Report-Only")}} HTTP headers are now supported. These allow websites to either enforce [subresource integrity guarantees](/en-US/docs/Web/Security/Subresource_Integrity) for scripts or only report violations of the policy, respectively.