Added definition of host language

Used existing verbiage to make a definition of "host language" while keeping all existing content in tact.

index.html

@@ -243,7 +243,12 @@ <h2>User Agent Support</h2>
 	</section>
 	<section id="co-evolution">
 		<h2>Co-Evolution of <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> and Host Languages</h2>
-		<p><abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> is intended to augment semantics in supporting languages like [[HTML]] and [[SVG2]], or to be used as an accessibility enhancement technology in other markup-based languages that do not explicitly include support for ARIA. It clarifies semantics to assistive technologies when authors create new types of objects, via style and script, that are not yet directly supported by the language of the page, because the invention of new types of objects is faster than standardized support for them appears in web languages.</p>
+		<p>A host language is a markup-based language that either:</p>
+		<ul>
+			<li>supports <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr>, using it to augment semantics; or</li>
+			<li>does not explicitly support <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr>, using it as an accessibility enhancement technology.</li>
+		</ul>
+		<p>For example, <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> augments semantics in supporting, host languages like [[HTML]] and [[SVG2]]. It clarifies semantics to assistive technologies when authors create new types of objects, via style and script, that are not yet directly supported by the language of the page, because the invention of new types of objects is faster than standardized support for them appears in web languages.</p>
 		<p>It is not appropriate to create objects with style and script when the host language provides a semantic element for that type of object. While <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> can improve the accessibility of these objects, accessibility is best provided by allowing the user agent to handle the object natively. For example, it's better to use an <code>h1</code> element in HTML than to use the <rref>heading</rref> role on a <code>div</code> element.</p>
 		<p>It is expected that, over time, host languages will evolve to provide semantics for objects that currently can only be declared with <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr>. This is natural and desirable, as one goal of <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> is to help stimulate the emergence of more semantic and accessible markup. When native semantics for a given feature become available, it is appropriate for authors to use the native feature and stop using <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> for that feature. Legacy content can continue to use <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr>, however, so the need for user agents to support <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> remains.</p>
 		<p>While specific features of <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> might lose importance over time, the general possibility of <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> to add semantics to web pages is expected to be a persistent need. Host languages might not implement all the semantics <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> provides, and various host languages can implement different subsets of the features. New types of objects are continually being developed, and one goal of <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> is to provide a way to make such objects accessible, because authoring practices often advance faster than host language standards. In this way, <abbr title="Accessible Rich Internet Applications">WAI-ARIA</abbr> and host languages both evolve together but at different rates.</p>