Use of css class instead of ID

Just wanted to check - I noticed that the wrapper and default templates use an ID to identify themselves. Shouldn't they be classes, otherwise you could end up with multiple ID's per page, which breaks HTML validation and potentially any custom code on the messages themselves?

Example scenario - you've got a message pop up as the page is refreshed and so is in the resulting source. On this page is a form, which fails validation on submit and there is an AHAH request returned which contains and additional message...