<sarcasm> ... </sarcasm>
The sarcasm tag is part of the old geek tradition of talking in pseudocode. It is part of fake HTML code, and as such is related to both fake C code and fake C++ code.
The ST has two main uses. It can be used in an email or IM conversation provided both users are geeks, or used while building the webpage of someone you hate.
First example:
Guess what?
What?
I'm getting married!
To whom?
Sussie-Lue!
<sarcasm>She's not ugly at all!</sarcasm>
Second example:
<p>Welcome to IRS.gov!
<sarcasm>We care what you think!</sarcasm>.
<br>To give us feedback <!-- and get yourself audited --> email us at feedback@irs.ustreas.gov.