The apos entity is an XML built it, and isn't defined for HTML. While some browsers support it in text/html documents, this is error correction and you should not use it.

Ah, that's interesting. I find it very useful to ensure that user-generated text doesn't break out of an HTML or JavaScript string, which is a big win IMHO. For example, if a template says:

<img src='$IMAGE1' alt='$DESCRIPTION1'>
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I can be sure that $IMAGE1 and $DESCRIPTION1 won't mess up my HTML formatting if I can ensure it doesn't have apostrophes, but otherwise it's impossible.

Are you aware of any browsers that don't support this entity in HTML?