webkit-gtk userscript h264ify (copied from https://github.com/erkserkserks/h264ify/blob/master/src/inject/inject.js , inspired by https://greasyfork.org/en/scripts/8128-youtube-h-264/code )

scripts.js

/*
 * webkit-gtk userscript h264ify
 * Licensed under The MIT License (MIT)
 * Original author: Copyright (c) 2015 erkserkserks
 */

function override() {
        // Override video element canPlayType() function
        var videoElem = document.createElement('video');
        var origCanPlayType = videoElem.canPlayType.bind(videoElem);
        videoElem.__proto__.canPlayType = makeModifiedTypeChecker(origCanPlayType);

        // Override media source extension isTypeSupported() function
        var mse = window.MediaSource;
        // Check for MSE support before use
        if (mse === undefined) return;
        var origIsTypeSupported = mse.isTypeSupported.bind(mse);
        mse.isTypeSupported = makeModifiedTypeChecker(origIsTypeSupported);
}

// return a custom MIME type checker that can defer to the original function
function makeModifiedTypeChecker(origChecker) {
        // Check if a video type is allowed
        return function (type) {
                if (type === undefined) return '';
                var disallowed_types = ['webm', 'vp8', 'vp9', 'av01'];
                // If video type is in disallowed_types, say we don't support them
                for (var i = 0; i < disallowed_types.length; i++) {
                        if (type.indexOf(disallowed_types[i]) !== -1) return '';
                }

                if (localStorage['h264ify-block_60fps'] === 'true') {
                        var match = /framerate=(\d+)/.exec(type);
                        if (match && match[1] > 30) return '';
                }

                // Otherwise, ask the browser
                return origChecker(type);
        };
}

override();