I heard someone use the term Old Skool when referring to the method of the past of writing poorly formatted and invalid HTML.

I think the proper term is deprecated. In fact, I think the term deprecated should be used more in everyday language. For instance, any time you are talking about something from the past, that is decidedly not cool, feel free to call it deprecated…

Basically: s/old skool/deprecated/g;

Example: “Check out that punk kid with the green mohawk, man, he’s deprecated!”