[css-module-bikeshed] Use pretty headers rather than header markup.

Author: dbaron

css-module-bikeshed/Overview.bs

@@ -33,16 +33,16 @@ Ignored Terms: div, dl, dfn
 }
 </style>
 
-<h2 id="intro">
-Introduction</h2>
+Introduction {#intro}
+=====================
 
 	<em>This section is not normative.</em>
 
 	<var class="replaceme">Provide background, motivation, etc.</var>
 
 
-<h2 id="sample-topic">
-Sample section</h2>
+Sample section {#sample-topic}
+==============================
 
 	Look at the mark-up in this section for examples.
 	See the <a href="https://www.w3.org/Style/Group/css3-src/bin/README">documentation</a> for more precise instructions.
@@ -102,8 +102,8 @@ Sample section</h2>
 
 	A formal definition of a property looks like this:
 
-<h3 id="the-display-model">
-Internal display model: the 'foo' property</h3>
+Internal display model: the 'foo' property {#the-foo-property}
+--------------------------------------------------------------
 
 	<pre class='propdef'>
 		Name: foo