Without realising it, for the last few days my XHTML has been invalid. While in pursuit of markup perfection, that will not stand.
The problem was that a blog entries title began with a number. That’s ok. However, those titles are used as ID values on the headers for that entry. ID values aren’t allowed to begin with numbers – I have no idea why, but that’s how it is.
The fix: prepend an X to the title.
The moral: just because an XML parser (my browser) didn’t notice the error, doesn’t mean that it’s valid XHTML. Validate, validate, validate.