[css-images] Fix the elementSources heading.

Author: tabatkins

css-images/Overview.html

@@ -119,8 +119,8 @@ <h2 class="no-num no-toc no-ref heading settled heading" id=contents><span class
 Bidi-sensitive Images</a></ul><li><a href=#image-set-notation><span class=secno>3.3</span>
 Resolution Negotiation: the <span class=css data-link-type=maybe title=image-set()>image-set()</span> notation</a><li><a href=#element-notation><span class=secno>3.4</span>
 Using Elements as Images: the <span class=css data-link-type=maybe title=element()>element()</span> notation</a><ul class=toc><li><a href=#paint-sources><span class=secno>3.4.1</span>
-Paint Sources</a><li><a href=#elementmap><span class=secno>3.4.2</span>
-The <code>ElementMap</code> interface</a><li><a href=#element-cycles><span class=secno>3.4.3</span>
+Paint Sources</a><li><a href=#elementsources><span class=secno>3.4.2</span>
+Using Out-Of-Document Sources: the <code>ElementSources</code> interface</a><li><a href=#element-cycles><span class=secno>3.4.3</span>
 Cycle Detection</a></ul><li><a href=#cross-fade-function><span class=secno>3.5</span>
 Combining images: the <span class=css data-link-type=maybe title=cross-fade()>cross-fade()</span> notation</a></ul><li><a href=#gradients><span class=secno>4</span>
 Gradients</a><ul class=toc><li><a href=#linear-gradients><span class=secno>4.1</span>
@@ -830,8 +830,8 @@ <h4 class="heading settled heading" data-level=3.4.1 id=paint-sources><span clas
 <p class=note>	Note: It is expected that a future version of this module will define ways to refer to paint sources in external documents,
 	or ones that are created solely by script and never inserted into a document at all.
 
-<h4 class="heading settled heading" data-level=3.4.2 id=elementmap><span class=secno>3.4.2 </span><span class=content>
-The <code>ElementMap</code> interface</span><a class=self-link href=#elementmap></a></h4>
+<h4 class="heading settled heading" data-level=3.4.2 id=elementsources><span class=secno>3.4.2 </span><span class=content>
+Using Out-Of-Document Sources: the <code>ElementSources</code> interface</span><a class=self-link href=#elementsources></a></h4>
 
 <p>	The <a class=css data-link-type=maybe href=#funcdef-element title=element()>element()</a> function normally selects elements within a document,
 	but elements that provide a <a data-link-type=dfn href=#paint-source title="paint source">paint source</a> don’t necessarily need to be in-document.
@@ -888,6 +888,11 @@ <h4 class="heading settled heading" data-level=3.4.2 id=elementmap><span class=s
 </pre>
 <p>		As the "fancy" canvas is drawn into and animated,
 		the backgrounds of all the H1 elements will automatically update in tandem.
+
+<p>		Note that the <a data-link-type=idl href=#dom-css-elementsources title=elementsources>elementSources</a> map is consulted <em>before</em> the document
+		to match the ID selector,
+		so even if there’s an element in the document that would match <span class=css data-link-type=maybe title=#fancy>#fancy</span>,
+		the backgrounds will still predictably come from the <a data-link-type=idl href=#dom-css-elementsources title=elementsources>elementSources</a> value instead.
 	</div>
 
 <h4 class="heading settled heading" data-level=3.4.3 id=element-cycles><span class=secno>3.4.3 </span><span class=content>

css-images/Overview.src.html

@@ -798,8 +798,8 @@ <h4 id='paint-sources'>
 	Note: It is expected that a future version of this module will define ways to refer to paint sources in external documents,
 	or ones that are created solely by script and never inserted into a document at all.
 
-<h4 id='elementmap'>
-The <code>ElementMap</code> interface</h4>
+<h4 id='elementsources'>
+Using Out-Of-Document Sources: the <code>ElementSources</code> interface</h4>
 
 	The ''element()'' function normally selects elements within a document,
 	but elements that provide a <a>paint source</a> don't necessarily need to be in-document.
@@ -860,6 +860,11 @@ <h4 id='elementmap'>
 
 		As the "fancy" canvas is drawn into and animated,
 		the backgrounds of all the H1 elements will automatically update in tandem.
+
+		Note that the <a idl>elementSources</a> map is consulted <em>before</em> the document
+		to match the ID selector,
+		so even if there's an element in the document that would match ''#fancy'',
+		the backgrounds will still predictably come from the <a idl>elementSources</a> value instead.
 	</div>
 
 <h4 id='element-cycles'>