ytspeed.js

function getCurrentVideo() {
    var videos = Array.from(document.getElementsByTagName("video"));
    videos.sort((a, b) => !b.paused - !a.paused);
    for (var v of videos) {
        if (v.paused && !videos[0].paused) {
            break;
        }
        var vbox = v.getBoundingClientRect();
        if (vbox.bottom > 0 && vbox.top < innerHeight) {
            return v;
        }
    }
    return videos[0];
}
function initDash() {
    var dash = document.createElement("div");
    dash.style.position = "fixed";
    dash.style.right = "0";
    dash.style.bottom = "0";
    dash.style.zIndex = "4999";

    var x = document.createElement("div");
    x.textContent = "❌";
    x.addEventListener("click", function () {
        dash.remove();
    });
    dash.appendChild(x);

    var range = document.createElement("input");
    range.type = "range";
    range.min = 0;
    range.max = 1;
    range.step = 0.1;
    range.value = 0.7;
    range.addEventListener("input", function () {
        var video = getCurrentVideo();
        video.style.filter = "brightness(" + this.value + ")";
    });
    dash.appendChild(range);

    var speed = document.createElement("div");
    for (var i of [1, 2, 3, 5, 10]) {
        var button = document.createElement("button");
        button.value = i;
        button.textContent = "x" + i;
        speed.appendChild(button);
    }
    speed.addEventListener("click", function (e) {
        var video = getCurrentVideo();
        video.playbackRate = parseInt(e.target.value);
    });
    dash.appendChild(speed);

    document.body.appendChild(dash);
}
initDash();