[selectors-4] Add case-sensitive attribute matching flag 's'. #2101

Author: fantasai

selectors-4/Overview.bs

@@ -181,6 +181,12 @@ Selectors Overview</h2>
 						exactly equal to any (ASCII-range) case-permutation of <code>bar</code>
 				<td>[[#attribute-case]]
 				<td>4
+			<tr>
+				<td><code>E[foo="bar" s]</code>
+				<td>an E element whose <code>foo</code> attribute value is
+						exactly and case-sensitively equal to <code>bar</code>
+				<td>[[#attribute-case]]
+				<td>4
 			<tr>
 				<td><code>E[foo~="bar"]</code>
 				<td>an E element whose <code>foo</code> attribute value is
@@ -1568,14 +1574,24 @@ Substring matching attribute selectors</h3>
 Case-sensitivity</h3>
 
 	By default case-sensitivity of attribute names and values in selectors
-	depends on the document language. To match attribute values case-insensitively
-	regardless of document language rules, the attribute selector may include the
-	identifier <code>i</code> before the closing bracket (<code>]</code>).
-	When this flag is present, UAs must match the attribute's value
+	depends on the document language.
+
+	To match attribute values case-insensitively
+	regardless of document language rules,
+	the attribute selector may include the identifier <code>i</code>
+	before the closing bracket (<code>]</code>).
+	When this flag is present,
+	UAs must match the attribute's value
 	case-insensitively within the ASCII range.
+
+	Alternately, the attribute selector may include the identifier <code>s</code>
+	before the closing bracket (<code>]</code>);
+	in this case the UA must match the value case-sensitively (if possible)
+	regardless of document language rules.
+
 	Like the rest of Selectors syntax,
-	the <code>i</code> identifier is <a href="#case-sensitive">case-insensitive</a>
-	within the ASCII range.
+	the <code>i</code> and <code>s</code> identifiers are
+	<a href="#case-sensitive">case-insensitive</a> within the ASCII range.
 
 	<div class="example">
 		The following rule will style the <code>frame</code> attribute when it
@@ -1586,11 +1602,28 @@ Case-sensitivity</h3>
 		<pre>[frame=hsides i] { border-style: solid none; } </pre>
 	</div>
 
+	<div class="example">
+		The following rule will style lists with <code>type="a"</code>
+		attributes differently than <code>type="A"</code>
+		even though HTML defines the <code>type</code> attribute
+		to be case-insensitive.
+
+		<pre>
+			[type="a" s] { list-style: lower-alpha; }
+			[type="A" s] { list-style: upper-alpha; }
+		</pre>
+	</div>
+
 	<!-- plinss notes we may eventually want to choose other normalizations
 			 for attribute matching; but since there's no normalization scheme
 			 that really seems needed at this point, this issue is deferred to
 			 a later level -->
 
+	Note: Some document models normalize case-insensitive attribute values
+	at parse time such that case-sensitive matching is impossible.
+	Case-sensitive matching via <code>s</code> flags is only possible
+	in systems that preserve the original case.
+
 <h3 id="attrnmsp">
 Attribute selectors and namespaces</h3>
 
@@ -3863,6 +3896,8 @@ Changes</h2>
 		(<a href="https://github.com/w3c/csswg-drafts/issues/1027">Issue 1027</a>)
 		<li>Dropped the '':drop()'' pseudo-classes since HTML dropped the related feature.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/2257">Issue 2257</a>)
+		<li>Added the case-sensitive flag <code>s</code> to the attribute selector.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/2101">Issue 2101</a>)
 	</ul>
 
 	Significant changes since the <a href="https://www.w3.org/TR/2013/WD-selectors4-20130502/">2 May 2013 Working Draft</a> include: