Add heading selectors (#3299)

Co-authored-by: Patrick Brosset <patrickbrosset@gmail.com>
Co-authored-by: Daniel D. Beck <daniel@ddbeck.com>

Author: 3 people

features/heading-selectors.yml

@@ -0,0 +1,7 @@
+name: Heading pseudo-classes
+description: The `:heading` and `:heading()` CSS pseudo-classes match headings of varying levels. The `:heading` pseudo-class matches any heading (`<h1>` through `<h6>`). The `:heading()` selector matches any of the given levels. For example, `:heading(1, 2)` matches headings level 1 and 2.
+spec: https://drafts.csswg.org/selectors-5/#headings
+group: selectors
+compat_features:
+  - css.selectors.heading
+  - css.selectors.headingfunction

features/heading-selectors.yml.dist

@@ -0,0 +1,9 @@
+# Generated from: heading-selectors.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support: {}
+compat_features:
+  - css.selectors.heading
+  - css.selectors.headingfunction