Dig it! I'm on a long HTML-learning journey as well. I think one of the main reasons that folks don't get this stuff is because the HTML Spec is so hard to read. I wrote a medium post about it: https://medium.com/weekly-webtips/how-to-read-the-html-spec-da54ca73513a

I want to learn more about the http-equiv attribute, I have been working in web dev for 7 years and haven't ever seen it used...