HTML1.1, HTML2, etc weren't less strict, if anything they were more. They only consumed tags that could be rendered, since there wasn't a concept of a DOM tree

HTML1.1, HTML2, etc weren't less strict, if anything they were more. They only consumed tags that could be rendered, since there wasn't a concept of a DOM tree or anything like that. They just worked forward rendering as they went. CSS and JavaScript introduced the need for a DOM tree and mutable state, in which case HTML renderers treated HTML docs more like XML descriptor files than SGML.