Nothing of this is invalid HTML. It has always been part of the HTML DTD that certain tags are optional and certain tags does not need to be explicitly closed.

Nothing of this is invalid HTML. It has always been part of the HTML DTD that certain tags are optional and certain tags does not need to be explicitly closed. This is a carry-over from SGML. The quoting rules for attributes are also well-defined, there is no guesswork or error-recovery involved. Now it this was about XHTML you would have a point. People often mix-up optional syntax and error recovery. You can leave out the <head>-tags, the parser can infer it - this is well-defined by the DTD. It is not an example of error recovery.