It can be a challenge to keep CSS files up to date as you change XSLT transformations to produce different HTML output. You've got to remember what you changed and then work out which CSS rules are affected and update them. Removing out-of-date rules is important, and can even affect your site’s ranking in search results, but it’s hard to do and requires attention to detail.
Recently I’ve been taking a different approach and I’m finding it much more productive.
