[einaregilsson]

No, it doesn't work on all builds of 10. I have Safari 11602.1.50.0.10, which is WebKit 602.1.50, it happens there at least.

[kip_]

I'm running macOS Sierra, 10.12 and Safari 10.0 (12602.1.50.0.10) I do not get the color change.

Yes, 12602, not 11602.