[selectors-4] Note that adherence to grammar is insufficient for vali…

…dity.

selectors-4/Overview.bs

@@ -3292,7 +3292,7 @@ Grammar</h2>
 	<dfn>&lt;pseudo-element-selector></dfn> = ':' <<pseudo-class-selector>>
 	</pre>
 
-	In addition to the above grammar,
+	In interpreting the above grammar,
 	the following rules apply:
 
 	* White space is forbidden between tokens except:
@@ -3307,6 +3307,11 @@ Grammar</h2>
 		with only a single ":" character at their start.
 	* In <<id-selector>>, the <<hash-token>>’s value must be an <a>identifier</a>.
 
+	Note: A selector is also subject to a variety of more specific syntactic constraints,
+	and adherence to the grammar above is necessary <em>but not sufficient</em>
+	for the selector to be considered valid.
+	See [[#invalid]] for additional rules for parsing selectors.
+
 	Note: The grammar above states that a combinator is optional
 	between two <<compound-selector>>s in a <<complex-selector>>.
 	This is only for grammatical purposes,
@@ -3319,15 +3324,13 @@ Grammar</h2>
 	if placed at the end of the last <<compound-selector>>
 	in a <<complex-selector>>.
 	In some circumstances, however,
-	it can be followed by more <<pseudo-element-selector>>s or <<pseudo-class-selector>>s
-	in the same <<compound-selector>>,
+	it can be followed by more <<pseudo-element-selector>>s or <<pseudo-class-selector>>s;
 	but these are specified on a case-by-case basis.
 	(For example, the <a>user action pseudo-classes</a> are allowed after any <a>pseudo-element</a>,
 	and the <a>tree-abiding pseudo-elements</a>
 	are allowed after the ''::slotted()'' pseudo-element)
 
 
-
 <h2 id='api-hooks'>
 API Hooks</h2>