Ah - well I guess the spec is written with the future in mind when everyone's got their servers configured correctly. Until then nobody has their servers set up right so I think it would be practical for browsers to be a bit more relaxed. Right now we're seeing a lot of games on servers without this MIME type, and the browsers respond by requesting all files from the server all over again every single page reload, probably wasting a lot of bandwidth and making HTML5 games look bad. I think there's a good case for a bit of relaxation on the browser side - standards compliance is good, but not if the web isn't ready yet.

No... just no. If browsers are relaxed now, then they are forced to provide backwards-compatibility for all the sites that then rely on those relaxed standards. Why do you think Internet Explorer is a complete mess and have dug themselves into a whole of having to ship half a dozen "compatibility modes" with every new version of their browser? Because they didn't stick to standards.