If you need to convert a WebM video file into a usable video format,
ffmpeg can take care of that for you.
ffmpeg -i input.webm output.mp4
If your file does not convert well, try this command:
ffmpeg -i input.webm -fflags +genpts -r 25 -movflags faststart -preset veryfast output.mp4
I needed these additional flags
-fflags +genpts -r 25 when I needed to convert a WebM from Ubuntu’s built in screen recorder. Before using these flags, the video played the first frame for the entire video duration. After, it played as expected.