It seems that Markdown is the way to write these days. If I'm writing anything for the web it's typically in MD, I'm even writing more and more general stuff in MD rather than turning to Word.
Rather then try to find web tools, I would buy a few of the many good content writing or copywriting and sales books out there.
Good copywriting doesn't come from automated grammar checkers and analysis tools. It comes from the personality and skill of a good writer!
But if you insist, you can find tools that will tell you if you're using uncommon words, or how high your reading level is for the audience, if your sentence structures are complicated, long, confusing, gender biased, whatever.
Anyway good luck!