[selectors-4] Split out pseudo-elements from <compound-selector>

Author: fantasai

selectors-4/Overview.bs

@@ -3513,13 +3513,14 @@ Grammar</h2>
 
 	<dfn>&lt;relative-selector-list></dfn> = <<relative-selector>>#
 
-
-	<dfn>&lt;complex-selector></dfn> = <<compound-selector>> [ <<combinator>>? <<compound-selector>> ]*
+	<dfn>&lt;complex-selector></dfn> = [ <<compound-selector>>? <<pseudo-compound-selector>>* ]!
+	     [ <<combinator>>? [ <<compound-selector>>? <<pseudo-compound-selector>>* ]!  ]*
 
 	<dfn>&lt;relative-selector></dfn> = <<combinator>>? <<complex-selector>>
 
-	<dfn>&lt;compound-selector></dfn> = [ <<type-selector>>? <<subclass-selector>>*
-	                        [ <<pseudo-element-selector>> <<pseudo-class-selector>>* ]* ]!
+	<dfn>&lt;compound-selector></dfn> = [ <<type-selector>>? <<subclass-selector>>* ]!
+
+	<dfn>&lt;pseudo-compound-selector></dfn> =  <<pseudo-element-selector>> <<pseudo-class-selector>>*
 
 	<dfn>&lt;simple-selector></dfn> = <<type-selector>> | <<subclass-selector>>