The more various skills you have the better, but do you need to know everything? No, of course not.
Do you need to know html to do SEO? It is fair to say that you really need to understand the basics of html – but, in no great detail. The ability to write some basic css rules is also worthwhile, but you don’t need to be a master of web design.
There’s some basic html document formatting attributes and properties you should become familiar with in order to be a great SEO.