One more step for bug 156536. I think when we get rid of all anonymous nodes it should be easier to fix that bug.
Currently this code is in RenderMathMLOperator and MathMLOperatorDictionary.
The operator properties depend on explicit attributes, on dictionary values (which depends on the infix/prefix/postfix form) and on the position of the operator inside its container value (for the form value):
Some are length values. See the related bug 118900.
Extending this to all operator attributes we support. Here are they:
All operator attributes are now parsed in element classes.