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In HTML 4 alt was required on all img elements, in html5 it is not required when The img element is in a figure element that satisfies certain conditions http://www.w3.org/TR/html5/embedded-content-0.html#guidance-for-conformance-checkers
https://github.com/whatwg/html-differences/commit/9968973743015dc0c1e653682522b22f201aaca9