A video compression mistake is almost always one of four things: choosing a source the browser cannot decode, picking a preset that doesn't match the destination, interrupting a real-time encode before it finishes, or trusting the size report before playing the resulting file back. None of these are exotic edge cases — they are the predictable consequences of how local, in-browser re-encoding actually works. The Video Compressor follows that model strictly: the file is decoded inside the current tab, drawn frame by frame onto a bounded Canvas, and recorded back to WebM using the browser's MediaRecorder. Because the encode observes the Canvas stream while the source plays in real time, every constraint — duration, resolution, codec support, preset — interacts with the others. Skipping a check in one stage tends to surface as an error, a larger file, or a silent loss of audio or metadata in the next. The mistakes below map to that workflow, not to a generic compression checklist, and they describe what the tool actually does rather than what a desktop encoder could do for you.
What Most Video Compression Mistakes Have in Common
Compressed-video failures cluster around the same handful of decisions: which file you hand the tool, which preset you pick, whether you let the real-time encode finish, and whether you actually play the download. Most mistakes are visible in advance if you know where the constraints live. The tool validates the file extension or MIME type, the byte size, the decoded duration, each dimension, and the decoded pixel area before it starts. That means a wrong codec, an over-size file, or a too-long duration is reported as an error rather than silently trimmed. The remaining mistakes happen after the encode starts: cancelling mid-stream, switching presets, closing the tab, or assuming the reported percentage means the result is acceptable without watching it. Treat each of those four checkpoints — input, preset, real-time run, verification — as a place a mistake can hide.
Before You Pick a File: Input Checks That Prevent Errors
The single most common compression mistake is feeding the tool a file it cannot decode. Browsers can play a long list of formats, but the underlying codec still has to be installed in the current tab. Renaming a file from .mov to .mp4 does not install a missing codec, and an H.265 source on a browser without an H.265 decoder will produce an error rather than a smaller file. Beyond the codec, the tool enforces a fixed set of input limits that bound memory use, Canvas allocation, and the real-time work the encode has to do. These limits are not configurable because raising any of them risks the browser tab running out of memory mid-encode.
| Limit | Value | Why it matters |
|---|---|---|
| Maximum file size | 500 MiB | Bounds memory while the source is loaded |
| Maximum duration | 300 seconds (5 minutes) | Bounds real-time encoding work |
| Maximum width or height | 4096 pixels | Bounded Canvas allocation per side |
| Maximum decoded area | 3840 × 2160 pixels | Hard cap on resolution |
| Required decoder | Browser-decodable | Container extension is not enough |
If any of these checks fail, the tool returns an error and no download is offered. That is by design: nothing is silently truncated, so an unsupported codec, an over-limit duration, an over-limit frame, a missing Canvas stream, or an unavailable WebM recorder produces a clear failure instead of a broken file.
Pick a Preset That Matches the Output You Want
Choosing the wrong preset is the next most common mistake. The three presets are bounded, transparent product choices rather than claims about a universal optimum. Each one caps the output width and sets a target frame rate, then asks a bounded bits-per-pixel heuristic for a recorder bitrate clamped between 180 kbps and 8 Mbps. The presets also preserve the source aspect ratio, never enlarge the source, and round output dimensions down to even pixel values so the codec accepts them.
| Preset | Max output width | Target fps | Bitrate range | Typical use |
|---|---|---|---|---|
| Small | 640 px | 24 | 180 kbps – 8 Mbps | Previews, low-bandwidth sharing |
| Balanced | 1280 px | 30 | 180 kbps – 8 Mbps | General sharing at HD |
| Quality | 1920 px | 30 | 180 kbps – 8 Mbps | Higher-fidelity playback |
The mistake here is assuming a higher preset always makes the file smaller, or that Quality is always "better." On a low-motion animation the Small preset can shrink the file substantially, while Quality applied to the same source wastes bitrate on pixels that never change. On noisy or shaky footage, any preset may produce a file similar to or larger than the source, because the codec has to spend bits preserving detail the eye cannot easily use. Pick the preset that matches the destination first, then check the result.
How to Compress a Video Locally Without Common Mistakes
- Pick a browser-decodable file no larger than 500 MiB, no longer than five minutes, and no wider or taller than 4096 pixels on a side, with a decoded area no larger than 3840 × 2160. Confirm the codec plays in your browser by opening the file locally before handing it to any tool.
- Open Video Compressor and choose the file. If the file fails the extension, size, duration, or dimension checks, the tool reports an error and produces no download.
- Select Small, Balanced, or Quality based on the destination. Small caps at 640 px and 24 fps, Balanced at 1280 px and 30 fps, Quality at 1920 px and 30 fps.
- Start compression and keep the tab open and focused while the source plays in real time. Encoding follows playback, so a two-minute source takes about two minutes rather than completing instantly.
- Do not choose a new file, switch presets, click cancel, or navigate away. Any of those actions invalidates the active job and releases the temporary object URL.
- When the source ends, read the result report: output dimensions, input bytes, output bytes, and the signed percentage change. Then download the WebM.
- Play the downloaded file fully, with sound, before deleting the original. Audio is only included when the browser exposes a capturable audio track; subtitles, chapters, and most metadata are not preserved.
Read the Result Report and Verify the Downloaded File
Another common mistake is trusting the percentage without watching the file. The tool computes the change from the actual Blob sizes of the input and the output, so a larger output shows up honestly as a positive number rather than being labelled a reduction. That is the most useful piece of information the report contains, because it tells you whether to try a different preset instead of accepting the result. After downloading, play the WebM end to end with sound on. Verify that duration, picture, and audio all match the source. If audio is missing, your browser may not have exposed a capturable audio track through HTMLMediaElement captureStream, which the tool can detect and add to the stream only when present. Do not assume muting the source tab has no effect on the captured track, and a browser that does not expose audio at all will produce a silent output regardless of preset.
Verification matters because the tool does not normalize volume, mix tracks, remove metadata according to a certified standard, repair corrupt media, or preserve subtitles, chapters, attachments, rotation tags, color metadata, or HDR signaling. WebM is the only output container because MediaRecorder support for browser-generated MP4 remains inconsistent across browsers, and the recorder tries VP9, VP8, then generic WebM in that order, so the same preset and source can produce slightly different files in different browsers. For a closer look at how to actually validate the result rather than just trust it, the verification workflow walks through the same checks in more detail.
When the Output Isn't Smaller: Try a Different Preset
Compression is content dependent, and a preset that shrinks one clip can grow another. A low-motion screen recording tends to compress dramatically under Small; the same preset applied to noisy handheld footage can leave the output near the input size because the codec spends bits on grain the eye cannot use. An already efficient source — for example, a clip that was previously exported by a good encoder — can produce a similar or larger file. The honest answer in those cases is to drop a preset rather than to argue with the result. If Quality returns a larger file, run Balanced or Small on the same source. The result you actually want comes from inspecting the download against the destination's requirements, not from the preset name alone. For more on exactly what those numbers mean, the accuracy guide explains how the byte-based percentage is computed. For archival masters, professional delivery specifications, exact codec profiles, two-pass rate control, fixed keyframe intervals, subtitle preservation, or deterministic cross-platform output, the tool is not the right job — a maintained desktop encoder such as FFmpeg with a reviewed command is. The browser compressor is for fast, local, privacy-preserving re-encodes where checking the result is part of the workflow, and where keeping the original until the download has been fully played is the single most reliable defence against a wasted re-encode.