I'm not a chemist, and I immediately understood this to mean "readily interacts/combines with multiple things", though my understanding did come directly from m

I'm not a chemist, and I immediately understood this to mean "readily interacts/combines with multiple things", though my understanding did come directly from my (limited, school-level) chemistry, so it's certainly domain-specific terminology that mightn't cross too well to web dev. Also, perhaps more importantly, it wasn't clear what things (rendering tech) the valencies were with until your comment.