> I can imagine a world where the first time my browser encounters a file of some new format it gives me the option to download a codec to render it That world

> I can imagine a world where the first time my browser encounters a file of some new format it gives me the option to download a codec to render it That world existed, in the 90s and early 2000s, and it was called "ActiveX". The codebase= attribute of the <object> element exists for that use case. It sucked, for two major reasons: first of all, it was a reliable way to get malware into user's machines (being "signed with a trusted cert" doesn't help when all you need for a trusted cert is money and/or hacking a trusted software publisher). Second, it was very Windows-specific, and a significant obstacle to the adoption of alternative operating systems, hardware architectures, and browsers.