Now try doing it when you close a tab/page.

IE11 is the last browser that can reliably send a request before the tab closes (by using synchrous).

I swear telling the business guys that we had no way to run an action when someone closed the page had them nearly running back to IE. And for that specific functionality, I find it hard to disagree.

In our case, we lock entities when someone starts working on them, but now we have no way to unlock them if they fat-finger the close button, or press one too many backspaces.