Please see this repro:

https://warp-nail.glitch.me <- browse to this url on iOS Simulator

https://glitch.com/edit/#!/warp-nail?path=index.html <- source

Note that the other ways to use this element are working on iOS, just not the for attr.

Probably has something to do with Safari's strict requirements for document.execCommand, as mentioned on this SO: https://stackoverflow.com/questions/34045777/copy-to-clipboard-using-javascript-in-ios