I’ve just had to fix the convert_smilies function in b2functions.php in order to maintain XHTML compliance.
Diffs are:

diff -r1.2 -r1.3
205c205
< $content = str_replace($smiley, "<img src='$smilies_directory/$img' />", $content);
---
> $content = str_replace($smiley, "<img src='$smilies_directory/$img' alt='' />", $content);

I’ve added an empty ALT attribute to the IMG tag.

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