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
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.
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.