Follow-up of bug 118053 comment 19.
The MathML code could be more consistent with what the CSS parsing does and could rely on some of the existing parsing functions (e.g. conversionToCanonicalUnitsScaleFactor from CSSPrimitiveValue.cpp).
This could help making LayoutTests/mathml/presentation/mspace-units.html pass on Mac.
I'm merging this into bug 156536 which includes more than just length attributes.
*** This bug has been marked as a duplicate of bug 156536 ***