HTML has about
142 tags!
Memorizing every single one of them isn’t so practical.
What’s important is that you remember the common ones that are used.
What’s most essential is that you understand how each of the tags work,
and how they relate to each other.
CSS is not easy. At times, you may try do so something,
and on paper it might seem like it will work, but in reality,
it ends up not working.
Mastering CSS requires an immense amount of practice.
Again, do not worry about memorizing every CSS property (there are
520 of them),but
make sure you understand how to apply them to achieve common goals.
Reference: https://www.mockplus.com/blog/post/learn-html-css