When is encoder conversion necessary?

The spec specifies that an implementation MUST support pixel format conversion for rendering and MUST support sample format conversion at least to float32. The spec thus makes it clear that everything has to be convertible to float32 (samples) and RGBA (pixels). But, all that VideoEncoder and AudioEncoder have to say on the matter is «Attempt to use [[codec implementation]] to encode frameClone according to options.». In practice, Chrome will happily convert anything to the pixel/sample format used internally by the selected codec. It'll even happily scale for you! But, it won't resample or mix/split tracks? Poking different AudioDatas and VideoFrames into it, I can get it to do some things and not to do others, but it's not at all clear what it will and won't do.

The question is, what aspects of this internal conversion during encoding are MUST, SHOULD, or implementation-specific? Right now, it seems like it's all implementation-specific, and if that's so, it would be nice to document that fact in the spec. But, surely it MUST at least support pixel and sample format conversion, as those are both codec-specific. The spec should make this clear.

(Apologies for all the issue posts lately, but the upshot is that there are now two implementations of the spec, so that's a good thing, right?)

Review the WebCodecs spec's VideoEncoder and AudioEncoder encoding steps, focusing on the instruction to encode a frame according to options. Compare the stated conversion requirements with the observed Chrome behavior, then clarify which pixel, sample, scaling, resampling, and channel-mixing conversions are required, recommended, or implementation-specific.