According to http://dev.w3.org/csswg/css3-flexbox/#algo-main-item : If flex item has an intrinsic aspect ratio, the flex item has ‘align-self: stretch’, the flex basis and cross size are both ‘auto’ , and the flex container is single-line and has a definite cross size, the flex base size is computed from the flex container's inner cross size (clamped to the flex item’s min and max cross size) and the flex item’s intrinsic aspect ratio. This was recently added to the spec, but I don't think we handle it properly.