Specifications should be a major part of the foundation we built on. Unfortunately, we're a bit loose with our adherence to specs. (Writer is guilty too).

While this was written before, I've decided to use it as a short illustration to #1109854 - Overly aggressive transliteration in drupal_clean_css_identifier on the difference between HTML and CSS with regards to 'allowed' characters.

Best to skip if you don't care about specifications ;)