[css-display] Describe SVG unboxing with element categories

Fixes #2118

Author: AmeliaBR

css-display-3/Overview.bs

@@ -35,6 +35,13 @@ spec:css-pseudo-4; type:selector;
 	text: ::first-line
 	text: ::before
 	text: ::after
+spec: svg2;
+    type:dfn;
+        text: container element
+        text: text content child element
+        text: renderable element
+        text: non-rendered element
+        text: rendered element
 </pre>
 
 <h2 id="intro">
@@ -1160,34 +1167,57 @@ HTML Elements {#unbox-html}
 SVG Elements {#unbox-svg}
 ---------------------------
 
-	: Root <{svg}>
+	: An <{svg}> element that has CSS box layout
+      (this includes all <{svg}> whose parent is an HTML element,
+       as well as document root elements)
 	::
 		''display: contents'' behaves as ''display: none''.
 
-	: Nested <{svg}>
-	: <{a}>
-	: <{g}>
-	: <{use}> (treated as a Shadow DOM container)
-	: <{tspan}>
-	: <{textPath}>
-	: unknown elements
+	: All other SVG [=container elements=] that are also [=renderable elements=]
+    : SVG [=text content child elements=]
+    : <{use}>
 	::
 		''display: contents'' strips the element from the formatting tree,
 		and hoists its contents up to display in its place.
+        These contents include the shadow-DOM content for <{use}>.
 
 	: any other SVG elements
 	::
 		''display: contents'' behaves as ''display: none''.
+        
+  <div class="note">
+  
+	The intention here is that 
+    the ''display: none'' behavior applies
+    whenever the "rendering context" inside the element
+    is different than the context outside of it.
+    If the element's child elements would not be valid children
+    of the element's parent,
+    you cannot simply hoist them up the formatting tree.
+    
+    For example, text content and text formatting elements in SVG
+    require a <{text}> element context;
+    if you remove a <{text}>,
+    its child text content and elements are no longer valid.
+    For that reason, ''display: contents'' on <{text}>
+    prevents the entire text element from being rendered.
+    In contrast, any content inside a <{tspan}> or <{textPath}>
+    is also valid content inside the parent text formatting context,
+    so the hoisting behavior applies for these elements.
+    
+    Similarly, if hoisting would convert the children from
+    [=non-rendered elements=]
+    (e.g., a shape inside a <{pattern}> or <{symbol}>)
+    to [=rendered elements=]
+    (e.g., a shape that is a direct child of the <{svg}>),
+    that is an invalid change of rendering context.
+    Never-rendered container elements therefore cannot be "un-boxed"
+    by ''display: contents''.
+    (Also, authors cannot override the 'display' property on these elements:
+    it is forced to ''none'' by an ''!important'' user agent style rule.)
+    
+ </div>
 
-	Note: The intention here is that "nested container" elements--
-	elements which have the same sort of "rendering context"
-	both outside and inside themselves,
-	such that it's <em>meaningful</em> to put their contents
-	into the context of their parents--
-	get stripped and hoist their contents like normal for ''display: contents''.
-	Anything else,
-	where their contents are handled in a totally separate context from their parent,
-	just removes itself entirely.
 
 MathML Elements {#unbox-mathml}
 -------------------------------