Add note to checkbox def (#1657)

Reference the native checkbox's strong native semantics / functionality and advise authors to use native features on the native element

Author: scottaohara

index.html

@@ -1990,6 +1990,9 @@ <h2>Definition of Roles</h2>
 			<div class="role-description">
 				<p>A checkable input that has three possible <span>values</span>: <code>true</code>, <code>false</code>, or <code>mixed</code>.</p>
 				<p>The <sref>aria-checked</sref> <a>attribute</a> of a <code>checkbox</code> indicates whether the input is checked (<code>true</code>), unchecked (<code>false</code>), or represents a group of <a>elements</a> that have a mixture of checked and unchecked values (<code>mixed</code>). Many checkboxes do not use the <code>mixed</code> value, and thus are effectively boolean checkboxes.</p>
+				<p class="note">
+					Due to the strong native semantics of HTML's native checkbox, authors are advised against using <code>aria-checked</code> on an <code>input type=checkbox</code>. Rather, use the native <code>checked</code> attribute or the <code>indeterminate</code> IDL attribute to specify the checkbox's "checked" or "mixed" state, respectively.
+				</p>
 			</div>
 			<table class="role-features">
 				<caption>Characteristics:</caption>