bitter pill: w3ctextarea ‘textarea xhtml validator’: Huh. This is a pretty good idea. Or you could just use Markdown.
w3ctextarea is a Firefox extension that allows you to validate data within a textarea as ‘XHTML 1.0 Strict’ using the w3c xhtml strict validator. It gleans any text from any and all textareas on the page, and validates said text with the w3c validator.