I have one Mac running High Sierra and another running El Cap. I also have an iPhone running iOS 11. All three devices use iCloud for Safari syncing, but for some reason iCloud tabs is broken on the El Cap Mac.
If I hit the iCloud tabs button it simply shows a blank bubble. On the HS Mac, open Safari tabs on both the iPhone and the El Cap Mac display, likewise on the iPhone I can see tabs open on both Macs. But for some reason the El Cap Mac's Safari will not display any open tabs on other devices. I've toggled Safari syncing on and off in the iCloud preferences on the El Cap Mac, but it didn't fix it. Any ideas?