The test in question is at testing/web-platform/tests/css/css-shapes/shape-outside/shape-image/shape-image-001.html Quote the spec [1], "The image is sized and positioned as if it were a replaced element whose specified width and height are the same as the element’s used content box size." So the shape image's width of <div id="image"> should be 150px, which leaves no room of the two 'X's. Google Chrome passes this test. I guess it uses the image's intrinsic width as the shape image's width on the <div> in this case. I doubt its correctness here. It however uses the element's content box if we change the <div> to <img id="image" src="support/left-half-rectangle-70.png"/>. [1] https://drafts.csswg.org/css-shapes-1/#shapes-from-image

We pass shape-image-001.html now, and Brad had done some fix in Bug 1404222 Part 8 https://hg.mozilla.org/mozilla-central/rev/1c17b43d22af