### What is the issue with the HTML Standard? The recently introduced [Reflect] etc covers a large portion of the uage of content attribute reflection, with the exception of two main things. Enumerated attributes that are limited to only known values (see #11455 for a solution), the other thing is reflection supporting ElementInternals (in practice I think this is only used on https://w3c.github.io/aria/#dom-ariamixin). We should solve the element internals use case so that the new IDL reflection can be used on the AriaMixin interface too. [ReflectSupportElementInternals] could work as a new primary reflection extended attribute but it's rather long winded? It's not immediately clear to me why this special flag is even necessary? I would have assumed it could be inferred by whether the attribute being reflected is on the ElementInternals interface (which these are via `ElementInternals includes AriaMixing`)