XHTML definition list gets stripped out on page save but not when clicking apply

Taken from this comment http://confluence.atlassian.com/display/DOC/Confluence+Storage+Format?focusedCommentId=282173689#comment-282173689

Edit a Confluence page, and open the source in the Source Editor plugin.

Insert an XHTML definition list. For example:

<dl>
<dt>Term 1</dt>
<dd>Definition 1</dd>
<dt>Term 2</dt>
<dd>Definition 2</dd>
</dl>

Click Apply to close the Source Editor plugin and return to the RTE.

The definition list appears, and you can edit its contents, but now save the page. The definition list disappears.

Edit the page again, and open the source in the Source Editor plugin. It shows an empty definition list element, <dl/>.