MacBookPro with Safari.

Safari hasn't supported >1080p in years (because it blocks VP9 and Youtube has stopped offering higher resolutions in H264 that Safari supports)

Blocks VP9? Maybe it doesn't support it but blocking would imply it is built-in but forcibly disabled.

It does look like they're testing it since it's in tech preview [1] so once it's relatively-bug free they'll probably release it.

[1] https://developer.apple.com/safari/technology-preview/releas...

Yup. Golden rule: don't use Safari for YouTube (or really for video content in general).