The HTML5 parsing algorithm is what the browser vendors want and need, however. Since HTML was never specified properly - not even in HTML4, where large areas were just undefined - HTML parsers have slowly evolved through trial and error. If big sites depend on a particular behaviour in a particular browser, other browsers have tried to be bug-compatible with that browser. Browsers that failed to do so have lost market share. If a browser decided to halt on encountering anything invalid would lose all of its users instantly, since the vast majority of documents are invalid.
Parsers have slowly converged towards each other, and the HTML5 algorithm is the compromise between them that breaks the last amount of content.
In other words, the goal of the HTML5 parsing spec was never to make something nice and clean. The goal was to define how to parse the unholy mess all you web developers out there have created during the past two decades. A clear spec, a good test suite and exactly identical behaviour between browsers is a benefit for everyone.