video-compressor: animated WebP output option

[simonw]

Add animated webp as an option to the video-compressor tool (alternative to mp4)

Claude Opus 5.5

A Format switch above the versions table picks MP4 video or animated WebP.

In WebP mode:

• Each version has a WebP quality from 0 to 100 (defaults 80/75/60/65/55) instead of CRF. The audio column and the MP4-only options (H.264 profile, no audio, strip metadata, limit to 30 fps) are hidden.
• Encoder speed sets libwebp's -compression_level: 0, 2, 4 (the default) or 6. In testing, 6 took about 10× as long as 4 for files only 1–2% smaller.
• Max frame rate defaults to 15 fps and never goes above the source's. File size grows almost in step with frame rate.
• Files are encoded with libwebp_anim, which stores only the parts of each frame that changed. They use -loop 0 because ffmpeg's WebP output plays only once by default. They have no audio, and the WebP muxer writes no metadata.
• Results preview as an <img> and download as .webp. "Encode full version" keeps the format.
• The embed snippet becomes '<img src="…webp" width height alt="" loading="lazy">', and the note above it says to fill in the alt text.
• libwebp_anim hands its output to ffmpeg only at the very end, so ffmpeg's progress time stays at zero. The progress bar counts the frame= log lines instead.

Results in both formats can sit side by side: switch format and generate again, and they're sorted by size. The poster frame is now extracted only before the first MP4 job, so a WebP-only run skips it.

Also adds a [hidden] { display: none !important } rule, so hidden elements with a CSS display value really hide. The poster row was showing while it was empty.

Animated WebP is much larger than MP4. The 3-second, 128 KB test clip came out at about 290–350 KB as WebP, and the UI says so.

Tests

Two new Playwright tests in tests/test_video_compressor.py:

• test_webp_format_switches_settings runs offline. It covers the controls, quality header, filename hint, the <img> embed snippet (including the XHTML variant), and that each version keeps its WebP quality and CRF separately.
• test_animated_webp_encode encodes a 1-second sample and then the full clip. It parses the RIFF chunks to check each file is animated, loops forever, is 640×360, has the right total length and has no EXIF/XMP/ICCP chunks. It also checks the command line, filenames and progress reporting, then encodes an MP4 alongside it.

pytest tests/test_video_compressor.py -k "not jsquash": 8 passed. test_poster_webp_falls_back_to_jsquash_without_native_support fails in the sandbox where this was written, on main as well: it fetches @jsquash/webp from jsDelivr inside the browser, and that browser had no network access.

🤖 Generated with Claude Code

https://claude.ai/code/session_01EGZFbqVeWgoceoCSNFnfvY

[claude]

Claude encountered an error —— View job

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I'll analyze this and get back to you.

[simonw]

WebP end up being a lot bigger than MP4, I'm going to land it anyway because it's an interesting demo of that fact.