[css-cascade] Clarify that the same stylesheet linked/imported in multiple places is still allowed to only be fetched once.

Author: tabatkins

css-cascade/Overview.html

@@ -326,6 +326,12 @@ <h2 id=at-import><span class=secno>2. </span> Importing style sheets: the
    multiple places, user agents must process (or act as though they do) each
    link as though the link were to a separate style sheet.
 
+  <p class=note> Note: This does not place any requirements on resource
+   fetching, only how the stylesheet is reflected in the CSSOM and used in
+   specs such as this one. Assuming appropriate caching, it is perfectly
+   appropriate for a UA to fetch a stylesheet only once, even though it's
+   linked or imported multiple times.
+
   <p> The <a href="#origin"><i>origin</i></a> of an imported stylesheet is
    the same as the <a href="#origin"><i>origin</i></a> of the stylesheet that
    imported it.

css-cascade/Overview.src.html

@@ -145,6 +145,13 @@ <h2 id="at-import">
 		user agents must process (or act as though they do) each link
 		as though the link were to a separate style sheet.
 
+	<p class='note'>
+		Note: This does not place any requirements on resource fetching,
+		only how the stylesheet is reflected in the CSSOM and used in specs such as this one.
+		Assuming appropriate caching, 
+		it is perfectly appropriate for a UA to fetch a stylesheet only once,
+		even though it's linked or imported multiple times.
+
 	<p>
 		The <i>origin</i> of an imported stylesheet is the same as the <i>origin</i> of the stylesheet that imported it.