Cut Video.

Cut it without re-encoding it. Quality is lost in the encoder, not in the cutting, so a tool that copies the existing packets into a shorter file cannot degrade anything. That is the default here. The trade is that the cut has to start on a keyframe, so your in point may land a fraction of a second earlier than you marked, keeping a little of what you wanted gone rather than losing any of what you wanted kept.

Because a lossless cut can only begin on a keyframe. Most frames in a video are stored as the difference from the frames before them, so they are meaningless without those frames; only a keyframe stands on its own. Cameras and screen recorders usually write one every one to three seconds, so your in point moves back by up to that much and keeps a moment of the dead air. Nothing you wanted is lost, which is why the default rounds this direction rather than the other. If the cut has to be on your exact frame, switch to frame-accurate and it will re-encode to get there.

Lossless, almost always. It is instant, it loses nothing, and half a second of extra head on a clip is not something anyone notices. Choose frame-accurate when the first frame matters: a cut on a beat, a clip that has to start on a specific word, or a trim you are handing to an editor who will match it against something else. That mode decodes and re-encodes the video, which takes real time and costs one generation of quality.

Not here, in the default mode. The video and audio packets inside your range are copied into a new container without being decoded, so the bytes that describe your picture are identical to the ones in the source. Every tool that re-encodes to trim, which is most of them, loses a little quality every time you use it. Frame-accurate mode is the exception in this tool and it is labelled as one.

Yes. MP4, MOV, MKV, and WebM are all read directly by the browser, and so are audio files like MP3, M4A, and WAV, which cut precisely because they have no keyframes to worry about. AVI is the exception: the browser cannot parse it at all, so run it through the converter first and trim the MP4 that comes out.

A lossless cut takes about a second, near enough regardless of how long the source is, because the work is reading and writing packets rather than looking at pictures. Frame-accurate mode is a real encode, so budget somewhere around real time for the section you kept: ten seconds of 1080p is a few seconds of work, and a ten-minute stretch is a few minutes.