[css-anchor-1] Specify that anchor-name only applies to elements that generate principal boxes, since anchor() relies on them doing so. w3c#7914

Author: tabatkins

css-anchor-1/Overview.bs

@@ -284,7 +284,7 @@ Name: anchor-name
 Value: none | <<dashed-ident>>
 Initial: none
 Inherited: no
-Applies to: all elements
+Applies to: all elements that generate a [=principal box=]
 Animation Type: discrete
 </pre>
 
@@ -295,8 +295,11 @@ Values are defined as follows:
 	:: The property has no effect.
 
 	: <dfn><<dashed-ident>></dfn>
-	:: The element is an <dfn dfn for local-lt="anchor">anchor element</dfn>,
+	:: If the element generates a [=principal box=],
+		the element is an <dfn dfn for local-lt="anchor">anchor element</dfn>,
 		with an <dfn dfn for>anchor name</dfn> equal to the <<dashed-ident>>.
+
+		Otherwise, the property has no effect.
 </dl>
 
 The [=anchor functions=]