W3 Schools W3Schools is a well-known tutorial site covering web technologies. Does not assume a lot of pre-existing background in these technologies.
MDN Web Docs The Mozilla Developer Network is another site that discusses the various web technologies. They tend to assume a bit more experience audience then W3 Schools, which sometimes mean they are more careful in the details, and sometimes mean they tell you lots of stuff you don't care about.
HTML 5 Test Visit the HTML 5 Test to get an idea how thoroughly your browser supports the HTML 5 standard.
Validator The W3C Validation Service checks to see if any particular page follows the correct HTML rules. Browsers tend to be forgiving. They just want to make your page work, and will ignore errors if the intent is clear. This won't. It's good to make your page compliant, if possible, because when browsers have to guess what you mean, they are likely to guess differently from each other, so your page will appear differently under different browsers.