[aapi] live roles

Hi,

There are some 'live' roles like "log" and "marquee". The Mozilla team
is considering these aria roles with respect to platform accessibility
API mapping.  One strategy is to not let these particular aria roles
override the native markup mapping. So something like

<table role="log">

would continue to map to whatever role the native/host language element
regularly mapped to (e.g. ROLE_TABLE).

Thoughts?

David