I have a messageboard and one of my users has written a greasemonkey script to stylize various elements within the page. He did a great job with it but in order to make his job easier, the first step in his script is to parse the current page and add several css classes to almost all html elements on the page. most of them aren’t used to style the page at all, but instead to make it easier for him to query the couple elements per page that he will actually modify. for example class="thread_started_by_user_123 thread_with_456_posts thread_with_789_views thread_last_posted_in_by_user_12345" etc etc
is this a standard practice? are there any downsides to adding lots of unnecessary css classes, either in javascript, or on the server if i were to add them to served pages too.
This looks to be using classes to embed arbitrary metadata into elements, which is certainly not what the class attribute was designed for. Given that its effects begin and end with a greasemonkey script and are thus localized to the client, it seems a harmless enough hack, but not one I’d advise duplicating on the server side.
HTML unfortunately doesn’t provide much in the way of alternatives when it comes to metadata other than sticking in invalid attributes, so there is a mechanism that does add semantic meaning to the “class” attribute within existing tags — namely microformats. There’s a lot of breathless buzzwordy hype around microformats, but overall they do revolve around a best practice for areas where going all-xml is out of the question.