[selectors-4] Whoops, attr-selectors allow whitespace between any of their tokens. Fixes #3151.

Author: tabatkins

selectors-4/Overview.bs

@@ -2240,14 +2240,14 @@ The Focus-Indicated Pseudo-class: '':focus-visible''</h3>
 	<em>and</em> the user agent determines via heuristics
 	that the focus should be made evident on the element.
 
-	
-	Note: The intent of '':focus-visible'' is 
+
+	Note: The intent of '':focus-visible'' is
 	to allow authors to provide <em>clearly identifiable</em> focus styles
 	which are visible when a user is likely to need to understand where focus is,
 	and not visible in other cases.
 
 	<div class="example">
-		In this example, 
+		In this example,
 		all focusable elements get a strong yellow outline on '':focus-visible'',
 		and links get both a yellow outline and a yellow background on '':focus-visible''.
 		These styles are consistent throughout the page and are easily visible
@@ -2276,40 +2276,40 @@ The Focus-Indicated Pseudo-class: '':focus-visible''</h3>
 	* If a user has expressed a preference
 		(such as via a system preference or a browser setting)
 		to always see a visible focus indicator,
-		the user agent should honor this 
+		the user agent should honor this
 		by having '':focus-visible'' always match on the active element,
 		regardless of any other factors.
 		(Another option may be for the user agent to show its own focus indicator
 		regardless of author styles.)
 	* Any element which supports keyboard input
-		(such as an <{input}> element, 
-		or any other element 
+		(such as an <{input}> element,
+		or any other element
 		which may trigger a virtual keyboard to be shown on focus
-		if a physical keyboard is not present) 
+		if a physical keyboard is not present)
 		should <strong>always</strong> match '':focus-visible'' when focused.
 	* If the user interacts with the page via the keyboard,
 		the currently focused element should match '':focus-visible''
 		(i.e. keyboard usage may change whether this pseudo-class matches
-		even if it doesn't affect '':focus''). 
+		even if it doesn't affect '':focus'').
 	* If the user interacts with the page via a pointing device,
-		such that the focus is moved to a new element 
+		such that the focus is moved to a new element
 		which does <em>not</em> support user input,
-		the newly focused element 
+		the newly focused element
 		should <strong>not</strong> match '':focus-visible''.
-	* If the active element matches '':focus-visible'', 
+	* If the active element matches '':focus-visible'',
 		and a script causes focus to move elsewhere,
 		the newly focused element should match '':focus-visible''.
-	* Conversely, if the active element does not match '':focus-visible'', 
-		and a script causes focus to move elsewhere, 
+	* Conversely, if the active element does not match '':focus-visible'',
+		and a script causes focus to move elsewhere,
 		the newly focused element should <strong>not</strong> match '':focus-visible''.
 
 	User agents should also use '':focus-visible'' to specify the default focus style,
-	so that authors using '':focus-visible'' will not also need to disable 
+	so that authors using '':focus-visible'' will not also need to disable
 	the default '':focus'' style.
 	</div>
 
 	<div class=example>
-	In this example, 
+	In this example,
 	authors use a '':not()'' pseudo-class to remove default user agent focus styling
 	if '':focus-visible'' is supported,
 	and provide enhanced focus styling via '':focus-visible''.
@@ -3445,6 +3445,7 @@ Grammar</h2>
 		* Around or in place of a <<combinator>>
 		* Between the parentheses and argument of
 			a functional <<pseudo-class-selector>> or <<pseudo-element-selector>>.
+		* Between any of the tokens of an <<attribute-selector>>.
 	* The four <a href="https://www.w3.org/TR/CSS2/selectors.html#pseudo-element-selectors">Level 2</a> <a>pseudo-elements</a>
 		(''::before'', ''::after'', ''::first-line'', and ''::first-letter'')
 		may, for legacy reasons,