My posts are usually notes and reference materials for myself, which I publish here with the hope that others might find them useful.

Given a source video source.mp4:

To compress for web at a reasonable broadband bitrate of about 1.5Mbps video / 128kbps audio:

ffmpeg -i source.mp4 -c:v libx264 -b:v 1.5M -c:a aac -b:a 128k target.mp4

To compress and also convert to WebM:

ffmpeg -i source.mp4 -c:v libvpx-vp9 -b:v 1M -c:a libopus -b:a 128k target.webm

(Note that the 33% lower bitrate -- 1M for VP9 vs 1.5M for H.264 -- is deliberate; VP9 encodes about 20-50% more efficiently than H.264. Opus and AAC are about equally efficient.)

To scale down a high-resolution source video to something more reasonable for Web (qHD for cellular, HD for broadband), the -filter:v argument is used:

ffmpeg -i source.mp4 -c:v libvpx-vp9 -b:v 0.33M -c:a libopus -b:a 96k \
-filter:v scale=960x540 target.webm

Command-line options: -c:v specifies the video codec; -b:v specificies video bitrate; -c:a specifies audio codec; -b:a audio bitrate; -filter:v applies a filter (in this case, scale) to the video stream.

To use the older VP8 codec with WebM, use libvpx instead of libvpx-vp9.

If the source video does not have an audio track, then omit the -c:a and -b:a arguments.

To explicitly specify the container format, use the -f command-line option: -f mp4 or -f webm.

For MP4 containers, the video codec used is H.264 and the audio codec is AAC, which enjoys near-universal browser support.

For Chrome and related browsers. WebM containers use VP8/9 and Opus.

Chrome also supports MP4 (H.264/AAC), so there is no point providing WebM unless the file size and/or quality is improved.

Bitrates can obviously be varied; I use 0.5M video and 96k audio for cellular.