check for direction differences in attribute values

[r12a]

If a tag has an attribute containing natural language text and element content that has a different direction, there is likely to be a problem indicating the base direction for the element. This should be pointed out.

needs some more consideration: for example, this may only apply if there is bidi text in one or both, since non-bidi should take care of itself wrt display – except perhaps where text is extracted and the paragraph base direction needs to be established (perhaps not a very common use case)