[css-conditional-4] Extend supports feature to express font capabilities

Defining the criteria for enabling @supports to distinguish support for
a set of font technologies defined in the grammar.

Addresses resolution in w3c#6520 to add criteria for font technologies as a
first step.

css-conditional-4/Overview.bs

@@ -34,8 +34,13 @@ Issue: In the future, copy the contents of [[css3-conditional]] into this docume
 This level of the specification extends the <<supports-feature>> syntax as follows:
 
 <pre class="prod def" nohighlight>
-	<dfn>&lt;supports-feature></dfn> = <<supports-selector-fn>> | <<supports-decl>>
+	<dfn>&lt;supports-feature></dfn> = <<supports-selector-fn>> | <<supports-font-technology-fn>> | <<supports-decl>>
 	<dfn>&lt;supports-selector-fn></dfn> = selector( <<complex-selector>> )
+  <dfn>&lt;supports-font-technology-fn></dfn> = font-technology ( <<font-technology>> )
+  <dfn>&lt;font-technology></dfn> = [ features( <<font-feature-technology>> ) | variations
+                      | color( <<color-font-technology>> ) | palettes | incremental ]
+  <dfn>&lt;font-feature-technology></dfn> = [ opentype | aat | graphite]
+  <dfn>&lt;color-font-technology></dfn> = [ COLRv0 | COLRv1 | SVG | sbix | CBDT ]
 </pre>
 
 : <<supports-selector-fn>>
@@ -44,6 +49,13 @@ This level of the specification extends the <<supports-feature>> syntax as follo
 	<a href="#dfn-support-selector">supports the selector</a>
 	provided as an argument to the function.
 
+: <<supports-font-technology-fn>>
+::
+    The result is true if the UA
+    <a href="dfn-support-font-technology">supports the font technology</a>
+    provided as an argument to the function.
+
+
 <h3 id="support-definition-ext">Extensions to the definition of support</h3>
 
 A CSS processor is considered to
@@ -52,5 +64,11 @@ if it accepts that selector (rather than discarding it as a
 parse error), and that selector doesn't contain
 <a>unknown -webkit- pseudo-elements</a>.
 
-<h2 class=no-num id="acknowledgments">Acknowledgments</h2>
+A CSS processor is considered to
+<dfn export for=CSS id="dfn-support-font-technology">support a font technology</dfn>
+when the text and layout processing engine ingesting
+this CSS text is capable of utilising the specific font-technology
+indicated by function and keyword
+in layout and rendering.
 
+<h2 class=no-num id="acknowledgments">Acknowledgments</h2>