Export serializable html5 metadata

My project uses html5 metadata exported from elements/html5.ts. But I'm having a difficuly in using it since it's uses callback to validate attribute's allowed (MetaAttributeAllowedCallback). MetaAttributeAllowedCallback consumes HtmlElement as function parameter, which is tightly coupled to html-validate library.

My proposal is to decouple metadata declaration with it's validation implementation, exporting the metadata as a plain object instead of object-with-behavior, so other libraries/apps can consume the metadata without caring about internal validation implementation of this library.

At the moment, I copy the metadata file and edit it manually (replacing the validation part) inside my project. But I think it would be better if I can import it instead.