I wanted to create a dummy text entry box in a comment so I tried using a form pointing nowhere, just to contain the text entry box. I wasn't sure which HTML elements were allowed, so I checked FAQ 72 beforehand. Since it said that <form> is OK, I thought I was fine.
However, the form didn't show up in the preview page. Looking at the source code, the HTML code for the form wasn't there, either (but it was still in the textarea below the preview area along with the rest of the comment).
I checked the FAQ again to see whether there was a difference between journal entries and comments, but no, it said unequivocally, "You can use any of the following HTML tags within a LiveJournal posting or comment: […] <form> […] <input> […]" (emphasis mine).
Trying it out in a journal entry of mine showed that the dummy form element showed up in the journal entry, but not in the comment. Presumable, <form> and/or <input> are not allowed in comments, then.
What are the HTML elements that are allowed in comments? Should this be documented in FAQ 72?