mattur - 5:53 pm on Sep 22, 2010 (gmt 0)
AIUI resizing an image via percentage scaling is seen as styling the image, so should be controlled via CSS, not HTML.
The <img>'s width and height attributes in HTML5 refer to the intrinsic dimensions of the image, and as you point out this is a change from previous (X)HTML specs which accepted n or n% for these attributes.
Does this change cause problems for you on your sites? May be worth raising with HTML-WG for discussion if it does.