Here's Henri Sivonen (the author of the HTML5 parser that is now Firefox' sole parser) on the subject: http://hsivonen.iki.fi/about-blank/ http://hsivonen.iki.f

Here's Henri Sivonen (the author of the HTML5 parser that is now Firefox' sole parser) on the subject: http://hsivonen.iki.fi/about-blank/ http://hsivonen.iki.fi/about-blank/. From a quick reading, it seems to be a mix of synchronous vs asynchronous parsing, historical reasons, and stuff. He identifies eight different behaviors.