Specify the format types that need to be supported

[jyasskin]

For maximal interoperability, the <model> spec should pick a single model format for browsers to all implement. (I'm not challenging the proposed API shape, with the <source type=...> parameter: that enables future extensibility.)

If there are already multiple formats with varying capabilities, such that none of them is strictly better than all the others, the specification should still list the types that may be supported and pick one lowest-common-denominator that all implementations must support.

If the browsers can't even agree on a lowest-common-denominator format, the specification should still list a set of formats that authors can provide that's guaranteed to cover all browsers. Arguably, it should require authors to provide options in all of those formats (by refusing to display the model if one of the <source>s is missing), but since authors could work around that by putting garbage URLs into the other <source>s, I'm not sure that's worth it.

I'm aware that the image and video elements fail to follow this advice: we shouldn't repeat that mistake for <model>.