netflix-video-speed.js

const video = document.getElementsByTagName('video')[0];

const slider = document.createElement('input');

slider.type = 'range';
slider.min = 0.5;
slider.max = 3;
slider.step = 0.25;
slider.value = 1;

document.body.appendChild(slider);

slider.style.position = 'absolute';
slider.style.zIndex = 9999;

const indicator = document.createElement('div');

indicator.textContent = '1x';

document.body.appendChild(indicator);

indicator.style.position = 'absolute';
indicator.style.zIndex = 9999;
indicator.style.right = 0;
indicator.style.fontSize = '30px';

slider.addEventListener('change', (e) => {
  video.playbackRate = e.target.value;
  indicator.textContent = `${e.target.value}x`;
});