[css-borders-4] Add 'hairline' keyword to <line-width>. w3c#3720

Author: tabatkins

css-borders-4/Overview.bs

@@ -379,12 +379,18 @@ Line Thickness: the 'border-width' properties</h3>
 	i.e. the <dfn export id="border-width-dfn">border width</dfn>.
 	Where
 
-	<pre class=prod><dfn><<line-width>></dfn> = <<length [0,∞]>> | thin | medium | thick</pre>
+	<pre class=prod><dfn><<line-width>></dfn> = <<length [0,∞]>> | hairline | thin | medium | thick</pre>
 
 	<p dfn-type=value dfn-for="<line-width>, border-width, border-top-width, border-left-width, border-bottom-width, border-right-width, border">
 	Negative values are invalid.
 	The <dfn>thin</dfn>, <dfn>medium</dfn>, and <dfn>thick</dfn> keywords
 	are equivalent to ''1px'', ''3px'', and ''5px'', respectively.
+	The <dfn>hairline</dfn> keyword is a UA-defined length,
+	less than or equal to ''1px''
+	and equal to an integer number of device pixels at the default page zoom,
+	which represents a "just visible" line.
+	(While it can be as large as ''1px'',
+	on many devices it will be approximately ''0.3px'' to ''0.5px''.)
 
 	The [=resolved value=] for the 'border-width' properties is the [=used value=].