mspace and CSS

cc @bfgeek @rwlbuis
This is related to CSS issues w3c/mathml#45 w3c/mathml#50 w3c/mathml#49 w3c/mathml-core#273

Rob has implemented the mspace element by mapping (in C++):

CSS width: width attribute value
CSS vertical-align: height attribute value
CSS height: calc(height attribute value+depth attribute value)

This has various consequences which make it different from the current specs and Gecko/WebKit implementations:

(1) One can now specify these CSS property directly on the mspace element and this will change its rendering and override the attribute values. The spec instead says they are ignored: https://mathml-refresh.github.io/mathml-core/#css-styling

(2) For mspace attributes, percentages are interpreted relative to the sizes of a block container. The spec instead says they are treated as 0: https://mathml-refresh.github.io/mathml-core/#space-mspace (this is consistent with MathML3 where the default size is 0x0)

Start by comparing the C++ mspace mapping described here with the MathML Core sections on CSS styling and space-mspace, then review the linked W3C issues and Gecko/WebKit behavior. Done requires a decided and documented resolution for CSS overrides and percentage handling, but this issue names no repository files or tests.