TenFiveTube is a standalone browser and player for using Youtube on PowerPC based Macs running OS X 10.5 Leopard.
You can follow the development of TenFiveTube on this Macrumors thread: https://forums.macrumors.com/threads/tenfivetube-yet-another-youtube-option.2103937/
TenFiveTube-v3.zip (55.97 MiB / 58.69 MB)
For optimal performance, you'll need Quicktime 7.7.
Emulating this? It should run fine under: QEMU