Remove "invalid rendering" for various MathML inputs.

[bfgeek]

See:
https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=6704
... for a simple example.

https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=6705
... for a more complex example.

The above example indicates that the implementations are trying to do something funny with various inputs which an implementation thinks is invalid.

I suggest removing these, and instead making sure that all the MathML algorithms can deal with the edge cases which developers might feed them.

(an alternative would be to strictly define, with algorithms, what type of DOM input is required, and the validation steps, however this would be prone to bugs, and compatibility issues).

This the a bit of a meta issue, however would need lots of tests with the "edge" cases for all of the different layout algorithms within MathML.

[fred-wang]

See issue w3c/mathml-core#136

[fred-wang]

closing this since it duplicates w3c/mathml-core#136