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/
(55.97 MiB / 58.69 MB)
(62.1 MiB / 65.12 MB)
/ DMG image
For optimal performance, you'll need Quicktime 7.7.
Emulating this? It could probably run under: QEMU