[css-pseudo-4] Clarify that highlight pseudos can cross element boundaries. Export terms.

Author: fantasai

css-pseudo-4/Overview.bs

@@ -423,8 +423,10 @@ Highlight Pseudo-elements</h2>
 <h3 id="highlight-selectors">
 Selecting Highlighted Content: the ''::selection'',  ''::inactive-selection'', ''::spelling-error'', and ''::grammar-error'' pseudo-elements</h3>
 
-  The <dfn>highlight pseudo-elements</dfn>
+  The <dfn export lt="highlight pseudo-element">highlight pseudo-elements</dfn>
   represent portions of a document that have been highlighted in some way.
+  They do not necessarily fit into the element tree,
+  and can arbitrarily cross element boundaries without honoring its nesting structure.
 
   <dl export>
     <dt><dfn>::selection</dfn>
@@ -639,7 +641,7 @@ Security and Privacy Considerations</h3>
 <h2 id="treelike">
 Tree-Abiding Pseudo-elements</h2>
 
-  <dfn lt="tree-abiding|tree-abiding pseudo-element">Tree-abiding pseudo-elements</dfn> always fit within the box tree.
+  <dfn export lt="tree-abiding|tree-abiding pseudo-element">Tree-abiding pseudo-elements</dfn> always fit within the box tree.
   They <a lt="inheritance">inherit</a> any inheritable properties from their <a>originating element</a>;
   non-inheritable properties take their <a>initial values</a> as usual.
   [[CSS3CASCADE]]