And that's really just the contribution of the <optgroup>'s label that needs to be ignored. If the <optgroup> is wrapping some <options>, those <option>s need to contribute to the intrinsic size calculations. (What about margin/padding/borders on the <optgroup> itself? Not sure, didn't test...)
Bug 1526855 Comment 2 Edit History
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And that's really just the contribution of the <optgroup>'s label that needs to be ignored. If the <optgroup> is wrapping some <option>s, those <option>s need to contribute to the intrinsic size calculations. (What about margin/padding/borders on the <optgroup> itself? Not sure, didn't test...)