About a year ago, Reddit share an over 1.75 years old Medium article on the designs and histories of X buttons -- "X's are everywhere in user interface (UI) design. A powerful symbol, [x] is capable of closing windows and popups, toolbars and tabs and anything else that might otherwise be cluttering up your screen.
Clicking on [x] to close a feature has become an instinctual part of using a computer and a standard in web and software design. Although it may seem like the ubiquitous [x] has always been a part of Graphical User Interfaces (GUI), a quick jaunt through the history of GUIs reveals that this actually isn’t the case.
So where and when did the [x] first enter into the UI lexicon? ..."
/me clicks on a X button.