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(values pretty-printed for legibility) """ Value: [ [ [ <percentage> | <length> | left | center | right ] [ <percentage> | <length> | top | center | bottom ]? ] <length> ] | [ [ [ left | center | right ] || [ top | center | bottom ] ] <length> ] """ http://dev.w3.org/csswg/css3-3d-transforms/#transform-origin The prose assumes that this works out to a three-dimensional point, but it's not explained how. The use of "?" in the first part and "||" in the second part implies that a two-argument version is allowed, but the meaning isn't explained. E.g., "50% 10px" is allowed by this grammar, but nothing says how to interpret it. In reality, it will be interpreted the same as "50% 10px 0", but I don't think that's the intent. I suggest that this be changed to """ Value: [ [ <percentage> | <length> | left | center | right ] [ <percentage> | <length> | top | center | bottom ] <length> ] | [ [ [ left | center | right ] && [ top | center | bottom ] ] <length> ] """ so that exactly three arguments must be provided for the three-dimensional version. Then if the 2D transform-origin is changed to only allow one or two arguments (bug 15432), there will be no ambiguity. (Alternatively, specify the one-, two-, and three-argument versions of transform-origin in the 3D spec, and have it supersede the 2D spec. Or merge the specs. But at a minimum, they should be consistent.)
*** This bug has been marked as a duplicate of bug 15432 ***