Castmate
Save an HLS (.m3u8) or DASH video as one MP4
An .m3u8 isn't a video file but a list of pieces. How to save it as a single MP4, and when you shouldn't.
Only save videos you have the right to keep: your own uploads, public-domain and openly licensed videos, or ones the site lets you download.
An HLS playlist (.m3u8) lists hundreds of short segments, often in several qualities, sometimes with sound in a separate list. DASH (.mpd) is similar. Saving it means picking one quality, downloading every segment in order and joining them, and joining the sound back in if it's separate.
With ffmpeg
ffmpeg -i "https://example.com/video/master.m3u8" -c copy video.mp4-c copy joins without re-encoding, so nothing is lost. If the server wants the page's address, add -headers "Referer: https://example.com/".
In the browser
Castmate (from version 1.0.5) does the same in Chrome: press Download under the video and choose where to save. It joins picture and sound, retries through short connection drops and shows the size when done.
What can't be saved
- DRM-protected streams, and AES-encrypted ones.
- Live streams (no end).
- Videos from sites whose terms forbid it, YouTube among them.