The converter has no resolution cap of its own; the limit is the upload size allowed on your plan, and the resolution stays as it is unless you choose 1080p, 720p or 480p. There is no HDR processing or tone mapping. When the video stream is copied (MKV to MP4 or MOV with the default settings), HDR10 passes through untouched. A re-encode drops the HDR10 mastering metadata, and with a Quality, Resolution or Frame Rate setting some pairs also reduce the video to 8-bit colour, so an HDR AV1 can look washed out.