It does make sense from the point of view of someone embedding HTML as a markup language, for users to edit documents online on a shared domain, and for tooling

It does make sense from the point of view of someone embedding HTML as a markup language, for users to edit documents online on a shared domain, and for tooling to be designed alongside these constraints. As it is, every piece of tooling has to define its own arbitrarily limited subset or an unlimited set of features (and associated attack surface).