A Custom Cartridge Makes YouTube Work on a Game Boy Color
Throaty Mumbo built a custom GBCTube cartridge to play YouTube video on a Game Boy Color's 160×144 display. The project progresses from displaying images and video to streaming over the link cable, then adds custom Wi-Fi-capable hardware.
The video demonstrates playback on real Game Boy Color hardware and covers the cartridge electronics, battery and enclosure built for the project.
Details: Throaty Mumbo ↗

There are countless easier ways to watch YouTube, but this one starts by building a Wi-Fi cartridge for a handheld that originally did not even have a backlit screen. The setup is more elaborate than the viewing.