Presto

Most of Presto's code exists because naive speed controllers break in three ways:

• Players reset the speed. Adaptive players reassign playbackRate on quality switches, ads and source changes, so every media element gets a guarded ratechange listener that puts your speed back.
• Every frame is isolated. Each iframe runs its own copy of the script. Speed changes are relayed to the top frame, which owns the state and broadcasts it down the frame tree.
• Sites bind the same keys. The key listener runs on window in the capture phase so it wins against player shortcuts, and it stays out of the way while you type in a field.

The control is drawn over each video instead of inside the player's markup, so it survives sites that rebuild their controls.

git clone https://github.com/SUD747/presto && cd presto
node test/unit.js
python3 -m http.server -d test 8000   # test page at http://localhost:8000/page.html

Then run the extension in a throwaway browser profile that reloads whenever a file changes:

npx web-ext run                                     # Firefox
npx web-ext run -t chromium                         # Chrome
npx web-ext run -u http://localhost:8000/page.html  # and open the test page

If you load it by hand instead, refreshing the page won't pick up edits to src/content.js. Reload the extension first.

• src/content.js: everything that happens on the page. Keep it a single file with no dependencies.
• src/popup.*: the toolbar panel. src/steps.js holds the step sizes both of them share.
• test/unit.js: speed and step arithmetic, extracted from content.js itself so the test can't drift from the code.
• test/page.html: a manual harness with a plain video, one in a shadow root, one in an iframe, a text field, and a video added after load.

• Add a case to test/unit.js if the change can be tested without a browser, or to test/page.html if it can't.
• Keep Mozilla's linter clean: npx web-ext lint --self-hosted should report zero errors and zero warnings.
• Say which real sites you tested on. Ad breaks and quality switches only show up there.

Found a security issue? Report it privately as described in SECURITY.md, not in a public issue.