Enhance metadata

  - Support escaping double quotes.
  - Support meta data for attributes.
Perhaps. You're also stating the reverse: If this value is anything but `true`, it's false -- and that's different than Javascript behaves. The real question is "Why would those be silently ignored?"

I wonder if it makes more sense to not have a boolean, but instead an indication of _how_ it should be escaped. `escape="attribute"`, `escape="rcdata"`, `escape="cdata"`, `escape="pcdata"`, `escape="html"`, for the various cases inside HTML.