Hide delete button using transparency rather than visibility

visibility:hidden apparently disables mouse events in Firefox, leaving the delete button rather useless!
There's a [test case here](http://codepen.io/ZackBoe/pen/Cuydk/), and the only bug report I could find was [this rather old one](https://bugzilla.mozilla.org/show_bug.cgi?id=711709).

It's also worth noting that the :after element won't display at all in IE11. (Thanks @simevidas)

I'm using Firefox Aurora 35.0a2 (2014-10-29)