Try this:
Code:
<html>
<body>
<idx:entry>
<idx:orth value="頭">
<idx:infl><idx:iform value="首"/></idx:infl>
</idx:orth>
<h3>頭 【あたま】首 【あたま】</h3>
【あたま; かしら; かぶり; こうべ; ず; つむり; つぶり; つむ; かぶ】<br />
(noun) (1) head; (2) mind; brain; intellect; (3) top; (4) hair (on one's head); (5) bangs; fringe; (6) top structural component of a kanji<br />
</idx:entry>
</body>
</html>
This defines 首 as a variant of 頭 and looking up either of these Kanjis will display the same entry.