w3schools is a wonderful website if you need to learn about HTML. It is very organized and has tons of information. However, sometimes the information can be overwhelming and hard to find a particular subject you need help on. Overall, I would suggest this site for help on HTML.
HTML Dog is a good website when you need to know something about simple things in HTML coding, such as lists or tables, etc. However, outside of that, it does not have that much information on advanced topics. I would not recommend this site.
EchoEcho is a really good website for HTML help. It has tons of information about many different topics. The information is very organized. Although sometimes it seems a little too huge, it is a great informative website, and I would recommend it.