From EKR: https://lists.w3.org/Archives/Public/public-webrtc/2017May/0166.html

The advice here (center, scale, then crop) differs from a MUST in JSEP S 3.6.

If the original resolution exceeds the size limits in the attribute,
the sender SHOULD apply downscaling to the output of the
MediaStreamTrack in order to satisfy the limits. Downscaling MUST
NOT change the track aspect ratio.