Define what writing-mode behaviour is desired.

[bfgeek]

I'm not sure what the best option here, and what use-cases there are, however here are three options.

1. Only horizontal-tb is supported.
2. Setting writing mode is valid on the top level element, and everything down to the , /etc elements is internall consistent. (This is similar to how tables work, i.e. all boxes from table->td are internally consistent).
3. Setting writing-mode is valid on any element.

Use-cases should guide the decision here. From my weak understanding math is typically only displayed in HTB, however I'm not a domain expert.

(2) May simplify implementations.

Any tests which are created for MathML should test that feature in each writing mode.

[fred-wang]

see also issue w3c/mathml-core#175

[fred-wang]

closing as it's very similar to w3c/mathml-core#175