Created attachment 154713 [details]
In the attached test case, the <svg> element ends up having a width of 300px (based on the default 300-by-150 for replaced elements with no intrinsic size) instead of 400px (based on its width attribute specifying 100%).
Created attachment 154726 [details]
Apply the rule for computing the width of elements with auto width and height, and intrinsic ratio but no intrinsic size to all such elements, regardless of whether they have a content renderer
Fixed in <http://trac.webkit.org/r123785>.