I'd say HTML.
The width and height attributes are used to stop the page from loading and growing in a disjointed manner.
If you have ever been reading a block of text on a page only to have it pushed downward thanks to the late loading image above you know how frustrating it is!
Adding an ID to every image just to specify its width and height would be ludicrous and just almost as messy as adding the two width/height attributes anyway.
The catch is Firefox not using the width and height attributes to reserve the images space however, Im surprised they haven't updated it actually.
