[css-images] Clarify that paint sources might not have intrinsic sizes.

Author: tabatkins

css-images/Overview.bs

@@ -332,8 +332,8 @@ Using Elements as Images: the ''element()'' notation</h3>
 Paint Sources</h4>
 
 	Host languages may define that some elements provide a <dfn export>paint source</dfn>.
-	Paint sources have an intrinsic width, height, and appearance,
-	separate from the process of rendering,
+	Paint sources have an intrinsic appearance and can obtain a <a>concrete object size</a>
+	without having to do layout or rendering,
 	and so may be used as images even when they're <a title='element-not-rendered'>not rendered</a>.
 
 	In HTML, the <code>&lt;img></code>, <code>&lt;video></code>, and <code>&lt;canvas></code> elements provide paint sources

css-images/Overview.html

@@ -418,8 +418,8 @@ <h4 class="heading settled" data-level=1.2.1 id=paint-sources><span class=secno>
 Paint Sources</span><a class=self-link href=#paint-sources></a></h4>
 
 <p>Host languages may define that some elements provide a <dfn data-dfn-type=dfn data-export="" id=paint-source>paint source<a class=self-link href=#paint-source></a></dfn>.
-	Paint sources have an intrinsic width, height, and appearance,
-	separate from the process of rendering,
+	Paint sources have an intrinsic appearance and can obtain a <a data-link-type=dfn title="concrete object size">concrete object size</a>
+	without having to do layout or rendering,
 	and so may be used as images even when they’re <a data-link-type=dfn href=#element-not-rendered title=element-not-rendered>not rendered</a>.</p>
 
 <p>In HTML, the <code>&lt;img&gt;</code>, <code>&lt;video&gt;</code>, and <code>&lt;canvas&gt;</code> elements provide paint sources