[css-highlight-api-1] Specify that custom highlights have no UA styles …

…#6375 (#6534)

* Specify that custom highlights have no UA styles, and inherit the styles of the originating element

* Lowercase 'c' in custom highlight pseudo-elements

css-highlight-api-1/Overview.bs

@@ -295,6 +295,13 @@ Applicable Properties</h4>
 	can only be styled with a limited set of properties.
 	See [[css-pseudo-4#highlight-styling]] for the full list.
 
+<h4 id=default-styles>
+Default Styles</h4>
+
+	UAs must not define any styles for [=custom highlight pseudo-elements=] in the default UA
+	stylesheet. A [=custom highlight pseudo-element=] inherits the styles of its
+	[=originating element=].
+
 <h4 id=c-and-h>
 Cascading and Inheritance</h4>