youtube-hd.js

// ==UserScript==
// @name        Youtube ensure HD
// @description Sets youtube player to highest resolution if drops from an HD res
// @match       *://www.youtube.com/watch*
// @match       *://youtu.be/watch*
// ==/UserScript==

let intervalID = setInterval(()=> {
  let qs         = (q)       => document.querySelector(q)
  let qsa        = (q)       => [...document.querySelectorAll(q)]
  let firstMatch = (q,p)     => qsa(q).filter(e => e.innerText.match(p))[0]
  let clickSeq   = (m,...ms) => m && (m = firstMatch(...m)) && (m.click(), setTimeout(clickSeq(...ms), 250))

  const adSelector                 = '.ytp-ad-player-overlay'
  const buttonWithBadgeSelector    = '.ytp-settings-button:is(.ytp-hd-quality-badge,.ytp-4k-quality-badge,.ytp-8k-quality-badge)'
  const buttonWithoutBadgeSelector = '.ytp-settings-button:not(.ytp-hd-quality-badge):not(.ytp-4k-quality-badge):not(.ytp-8k-quality-badge)'
  
  if (qs(adSelector)) return                  // don't try anything during ads
  if (!qs(buttonWithoutBadgeSelector)) return // don't try if we can't find a settings button without a badge (ie not present somehow, or has badge)

  console.log("Resetting youtube to HD")

  clickSeq(
    ['.ytp-settings-button', /()/],
    ['.ytp-menuitem',        /^Quality/],
    ['.ytp-menuitem',        /^(4320p|2160p|1440p|1080p|720p)/])

  // check that we got the hd badge or else give up, probably not hd video
  setTimeout(()=> {
    if (qs(buttonWithBadgeSelector)) return
    clearInterval(intervalID)
    console.log("HD content not set, aborting further attempts")
  }, 600)

}, 1000)