Most of the videos, movies, TV shows, trailers, live streams hosted on YouTube is now available in ultra high definition (UHD) and high definition (HD) format, such as 8K (4320p), 5K (288p), 4K (2160p), 1440p, 1080p, and 720p. However, when viewing the video playback, YouTube may choose to play the video at the SD resolution of 480p, 360p, 240p or 144p, despite the source is available in HD resolution.
YouTube adjusts the playback quality of video stream between standard definition (such as 240p or 360p) and high definition (720p or 1080p) based on the speed of the Internet connection (bandwidth), and almost never in 4K UHD or above quality automatically.
YouTube viewers can always change the video quality in Settings of the video player, however, it’s annoying to have to change the playback quality if YouTube is unable to detect your available Internet bandwidth properly. YouTube no longer provide options to set preferred video quality during playback. Nonetheless, it’s possible to use extensions, add-ons or plugins on web browsers to watch YouTube automatically not only in HD, but in highest resolution possible.
There are plenty of extensions for Chrome that can help users to play YouTube videos in HD or UHD automatically. One example is Turn Off the Lights, which among its powerful features for YouTube is ability to set the quality of YouTube videos automatically.
Several add-ons can be used to automatically play YouTube videos in highest available HD or 4K / UHD quality such as Enhancer for YouTube, YouTube High Definition, Auto HD|LQ for YouTube, YouTube in HD (720p, 1080p, 1440p, 2160p) and more
Most of the extensions and plugins to play YouTube in better HD or UHD quality available in Chrome and/or Firefox is also available for Opera web browser.
Try Turn Off the Lights and High Definition Quality for YouTube.