If you're going to do it that frequently then it probably makes sense to look at the process that creates/updates the HTML in the first place. Why not make it well-formed XHTML, then you can process it directly. Even better, make it XML tagged with logical markup rather than presentation markup.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference