No. It goes only because there are very few browser engines, who mostly can align their behaviour with each other.
Protocols with miltiple implementations are way more strict, because you can't feasibly test your quirky approach on every implementation, and the chance they will all be as forgiving is slim.
The parsing is easy. And done in many, many libraries outside of the browsers. (And no, parsing with regex is still not possible, zalgo)
The problem is not parsing of HTML, its the DOM events, CSS application, javascript apis and above all the combination thereof which must be rendered all exactly the same, what makes it hard.
