It reminds me strongly of PDF and Acrobat. PDF is great for mailing around print-ready documents, which are more-or-less guaranteed to look the same for every viewer. Writing a PDF renderer is not easy, but it is straightforward, and there are multiple stable implementations without significant security problems.
Then Adobe comes along and they add forms, and 3D charts, or Javascript, or multimedia, and Acrobat grows from a document viewer into what is essentially a backdoor on every Windows computer.
A similar thing is happening with browsers. The core purpose of a web browser is the ability to render HTML+CSS into a human-readable document. Then browser vendors added forms and Javascript, so XSS was invented. They added persistent data storage, so looking at cat pictures can compromise my bank account. And now, Chrome+Firefox are /competing/ to see who can add more features, security be damned.
WebGL exposes your graphics drivers (never security-audited before) to the internet. <audio> and <video> expose multimedia codecs, which in the past have caused numerous security problems. Flash is, essentially, a cross-platform way to let arbitrary people run exploits on your machine.
When will it stop? When will browser vendors take a collective breath, look around, and realize the insanity they've been perpetrating?