I'd recommend starting off withw3schools. I'm not a web developer, however, I had to study html and css cause my school programme forced me to and I used w3s as my primary learning resource. I also heard that codeacademy is pretty good. I went through their html course and they seem legit. Obviously, you won't learn anything without actually using it so I suggest that you try implementing everything on your learning website as you go. You should google any problems that you have, and if you can't find the answer then I'm sure the people on here would be more than happy to help!