The img lowsrc attribute was deprecated a long time ago. If content negotiation isn't going to be used, then a system like that, with separate attributes for the different URLs, makes more sense than overloading the contents of the current src attribute with a custom, differently parsed format (explode on commas or whathaveyou).
The comma delimited list of file extensions was an illustration of a bad solution. The actual proposal was to use proper content negotiation and the Vary header for intermediate caches.
