WebM to MP4: test compatibility where the recipient will watch
An application or recipient asks for MP4 rather than WebM. This is a delivery-compatibility requirement, not necessarily evidence that the WebM is broken. Separate an accepted extension from a correctly re-encoded clip.
MP4 does not guarantee every kind of playback
A container holds video and audio tracks; their internal encodings also affect compatibility. Changing an extension to .mp4 therefore does not repair a file that has not actually been converted.
Try the result in the intended device or application before sending a large set. Playback in another viewer is useful evidence, not acceptance on behalf of every destination.
References: MDN — Media containers and their tracks
Check portrait video as carefully as landscape video
The current path preserves aspect ratio within 1280 × 720 and produces 30 frames per second. Its height limit means a portrait video may be much narrower than 1280 pixels; it should not stretch to fill a landscape rectangle.
Some destinations crop their previews. Open the actual file to distinguish a cropped interface thumbnail from cropping in the downloaded output.
Playback compatibility is not a promise to retain every extra
This is a basic video-and-audio copy, not a promise to transfer every subtitle track or extra metadata field. Unsupported tracks may make a source unsuitable for this conversion. Keep the original when those extras matter.
Inspect the beginning and ending of speech and a clear visible action. Similar displayed durations alone do not prove synchronization. Compare with the source before attributing a problem to it.
Example: a portrait clip inside the output boundary
A 480 × 800 clip needs a factor of 720 ÷ 800 = 0.9 to fit the 720-pixel height. It becomes 432 × 720. The ratio remains 0.6 in both cases: 480 ÷ 800 = 432 ÷ 720.
Side bars in a wide player may correctly display a portrait frame rather than signal a failure to fill the screen. Stretching that picture to 1280 pixels wide would distort it instead of fixing it.