In the November 5 Editor’s Draft of HTML5: A vocabulary and associated APIs for HTML and XHTML, there is a curious reference to the “sarcasm” tag.
8.2.5.4.7 The “in body” insertion mode
When the user agent is to apply the rules for the “in body” insertion mode, the user agent must handle the token as follows:
…
An end tag whose tag name is “sarcasm”Take a deep breath, then act as described in the “any other end tag” entry below.
This is the only reference to the tag, so I guess only the closing </sarcasm> tag is allowed, not the opening <sarcasm> tag.
Perhaps this was a test to see if anyone’s actually reading?
Mad File Format Science 
I suspect that this isolated closing tag is more often used than any other (obviously in non-HTML text, or should be). I can’t prove it — Google, Yahoo search, and Bing all ignore the non-alphabetics when told to search for — but that’s how ISTM.
Partly, I guess, because sarcasm (or irony, which many people confuse it with) can lose a lot of its force if you warn about it ahead of time. But … heh, ESPECIALLY if you think the word means “irony” … the isolated closing tag is a way to say “Hey, don’t take that seriously, I didn’t mean it.”
There is, however, a serious reference to sarcasm, or perhaps I should say against it for HTML:
And here I hoped they were talking about quadium…