dom.js

Not really. Your first paragraph is also true for JS (think of ES6 "class" syntactic sugar). Some browsers implement the DOM in JavaScript (https://www.chromium.org/blink/blink-in-js, http://www.phantomjs.org/, https://github.com/andreasgal/dom.js). And there are things like asm.js and even Linux VM running in JavaScript (http://bellard.org/jslinux/). JavaScript/ECMAScript is one of the most misunderstood languages, please read up before jump at conclusions.

Interestingly, there are also a prototype Flash in JavaScript called Shumway: https://github.com/mozilla/shumway And an IIUC a DOM renderer in JS https://github.com/andreasgal/dom.js/ I don't know if PDF.js will ever reach full PDF compatibility (with all it's weird forms and other stuff), but I do hope so. With a liberal licence I can totally see people embedding pdf.js to render their pdfs on site.

Some day was two years ago: https://github.com/tmpvar/jsdom https://github.com/andreasgal/dom.js

Right, WebKit shipped code based on the spec first but the spec itself underwent subsequent revision as the Gecko / Presto implementors found site compatibility issues and bugs. I think the WebKit implementation was recently updated to the spec, so we are at, or at least very close to, having very interoperable HTML parsing in Opera/Firefox/Safari/Chrome. I also believe that Microsoft are aiming to implement the new algorithm in IE 10. From an interoperability point of view the HTML parsing algorithm is the poster child for the success of the HTML effort; there is a testsuite of several thousand tests [1] (also submitted to the W3C [2]) that has contributions from multiple browser vendors and a number of unaffiliated individuals. Although parsing isn't sexy in the way that, say, <canvas> is, getting interoperable parsing makes it much eas