Update undefined definition, add assignment examples

index.html

@@ -10013,7 +10013,7 @@ <h3>Value</h3>
 				<dt id="valuetype_tristate">tristate</dt>
 				<dd>Value representing <code>true</code>, <code>false</code>, <code>mixed</code>, or <code>undefined</code> values. The default value for this value type is <code>undefined</code> unless otherwise specified.</dd>
 				<dt id="valuetype_true-false-undefined">true/false/undefined</dt>
-				<dd>Value representing <code>true</code>, <code>false</code>, or <code>undefined</code> (not applicable). Unless otherwise specified, the default value for this value type is <code>undefined</code>. Note that the value <code>undefined</code> conveys a state- or property-dependent behavior including but not limited to lack of support (e.g., <sref>aria-checked</sref>, <sref>aria-pressed</sref>), equivalence with the value <code>false</code> (e.g., <sref>aria-multiselectable</sref>), behavior determined by the user agent (e.g., <sref>aria-hidden</sref>) or non-applicability (e.g., <sref>aria-selected</sref>).
+				<dd>Value representing <code>true</code>, <code>false</code>, or <code>undefined</code>. Unless otherwise specified, the default value for this value type is <code>undefined</code>. The value <code>undefined</code> (not the string "undefined") usually represents the absence of an state or property in HTML (e.g., <sref>aria-expanded</sref> is not present on an element), or explicit assignment of an IDL attribute via JavaScript (e.g., <code>el.ariaExpanded=undefined</code>). Note that the value <code>undefined</code> conveys a state- or property-dependent behavior; for example, an element with <sref>aria-expanded</sref> set to <code>undefined</code> is not expandable. Where <code>undefined</code> is a supported value, consult the values table of the relevant state or property to understand its meaning.
 				<dt id="valuetype_idref">ID reference</dt>
 				<dd>Reference to the ID of another <a>element</a> in the same document</dd>
 				<dt id="valuetype_idref_list">ID reference list</dt>