I'm embarrassed that I'm confused enough (ignorance about how webpages and javascript modern things, etc work) to not answer this on my own, but does this mean:
1) Beware! any site you visit can, via javascript, inject whatever it wants into your clipboard (write access)
2) Beware! any site you visit can, via javascript, do whatever it wants with your clipboard (READ and write access) 3) some combination of the above.
4) something different.
I use exclusively Firefox (as opposed to any browser) and haven't touched wayland. I also only type on keyboards with a trackpoint and a middle click button, and have for the past few decades. Hence, I'm sure I make heavy use of all the various methods of accessing clipboards..
(I'm guessing it's closer to #1 than #2, I think I once saw a crypto address in my clipboard unexpectedly on a VM...)