1. Change the servlet logic so that by default we no longer do doctype sniffing of text/html documents to determine what schema to check them against. e.g., for documents with an HTML4 doctype, we have by default been validating them checking them against the old schema for HTML4. But now, instead, we'll just validate all text/html documents against the latest HTML schema (aka "HTML5").

2. Provide a new "sniffdoctype" option in the REST API to allow maintainers of third-party services or custom front-ends to opt back into the old doctype-sniffing behavior if they need to -- to preserve the possibility of backward compatibility in case the lack of that option would break expectations the providers of those services may have already set for their existing users.