[css-values] Add x unit, make it the canonical unit.

Author: tabatkins

css-values/Overview.bs

@@ -1408,6 +1408,7 @@ Resolution Units: the <<resolution>> type and ''&lt;resolution&gt;/dpi'', ''&lt;
 		<dd>Dots per centimeter.
 
 		<dt><dfn id="dppx">dppx</dfn>
+		<dt><dfn id="x">x</dfn>
 		<dd>Dots per ''px'' unit.
 	</dl>
 
@@ -1417,7 +1418,7 @@ Resolution Units: the <<resolution>> type and ''&lt;resolution&gt;/dpi'', ''&lt;
 	or the 'image-resolution' property defined in [[CSS3-IMAGES]].
 
 	All <<resolution>> units are <a>compatible</a>,
-	and ''&lt;resolution&gt;/dppx'' is their <a>canonical unit</a>.
+	and ''&lt;resolution&gt;/x'' is their <a>canonical unit</a>.
 
 	<div class="example">
 		The following @media rule uses Media Queries [[MEDIAQ]]