How to edit a video
Drop in an MP4, MOV, WebM or GIF. Drag the two handles on the strip of frames to keep only the part you want, or type exact times into Start and End; the play button plays just that part. Pick a Crop shape and drag the box over the picture to frame it. Then choose a size, a speed and whether to keep the sound, and press Save video or Make a GIF.
What comes out
Save video writes an MP4 with H.264, which plays everywhere, when your browser can encode H.264; otherwise, and whenever you pick it, a WebM with VP9 (or VP8 on older browsers). The quality slider sets the bitrate, and the line under it estimates the file size before you start. Make a GIF turns up to 15 seconds into an animated GIF at 10, 15 or 20 frames a second and up to 640 pixels wide. Every frame is decoded and encoded again, so the result is a new file, not a copy with the ends cut off.
Private by design
The video is decoded, edited and encoded in this page with your browser's own video codecs (WebCodecs). It is never uploaded, so it is safe to edit something you have not shared yet. Close the tab and it is gone.
Questions
Which browsers work?
Chrome, Edge and other Chromium browsers, and Safari 16.4 or later. Firefox does not have the video encoder this needs yet; the page says so rather than failing halfway. Which formats you can open depends on the browser too: most open H.264, VP8 and VP9, but H.265 (HEVC) from newer iPhones only opens where the browser can decode it.
Is there a size limit?
The whole file is held in memory while it is edited, so a file over about 1.5 GB is turned away with a reason rather than crashing the tab. Long clips are fine otherwise: frames are processed one at a time, not all at once. It is faster to keep a short part than to re-encode a whole film.
Why is there no sound at 0.5× or 2×?
Changing the speed of sound changes its pitch unless it is stretched carefully, which this tool does not do. So at any speed other than 1× the sound is left out, and the page says so. GIFs never have sound.
Why did my MP4 come out as WebM?
Your browser can't encode H.264, so the MP4 choice is turned off. Some Linux builds of Chromium lack it. WebM plays in every modern browser and in VLC.
Will it lose quality?
A little, as any re-encode does. Keep the quality slider toward the right for a clip you will edit again, and toward the left to make a small file to send.