The image ends up left-aligned. I assume that the issue is that
nsBlockReflowContext::AlignBlockHorizontally has been removed and the replaced
element cases in nsHTMLReflowState can't call CalculateBlockSideMargins without
calculating a useful mComputedWidth first, right?
Perhaps we should add a GetIntrinsicHeight() method on nsIFrame (returns 0 by
default, overridden by things that have an intrinsic height) and try to actually
implement the intrinsic-sizing algorithms CSS2.1 describes?