As Deestan said, there's a massive difference between how low level assembly code is, and how high level HTML is. But even more than that is the fact that they're fundamentally different. Decomposing a sequential list of instructions down into machine code is something that a is fairly simple to do algorithmically. But HTML is not a programming language - it is a descriptive instead of declarative or imperative.

In the end, humans are a lot better at describing things than algorithms, and I think that's the biggest problem with WYSIWYG HTML editors.