pecc wrote:ALT text isn't supposed to be a pop-up tip, and the browsers are simply functioning as they should. (It is a wide-spread misunderstanding.) TITLE is the one that should create a pop-up.
It was a widespread misunderstanding a number of years ago (particularly when Internet Explorer was more dominant), but most people have figured that out by now, or so I would hope. xkcd.com certainly stores pop-up text in title tags.
The most simple and probably best solution would be that the pop-up texts for the strips were entered in both the ALT and TITLE fields in the future to ensure their visibility in most browsers. Old strips could be updated with a simple script.
Most people are also using CSS or other means of generating pages dynamically rather than having static HTML for every single page.
This is why bumping a year-old thread is not always a good idea.