Play/Pause in Mojave Safari

Regular goofing-off workflow: Playing music in iTunes. Use Feedbin to find blog posts with video, song, or podcast I want to play. Open these in tabs. Pause iTunes with fn-F8. Click on the play button on a media page.

Except now, that's not what happens. The media control buttons will affect Safari, and iTunes keeps going. And since TunesArt doesn't work, I have no easy way to tell iTunes to shut up except to tab into it, find the pause control, then back to Safari where the video's still going.

I don't see any way in Safari or iTunes or System Preferences to tell it "media buttons only control iTunes". INFURIATING.