The popular Close button, ×, originated from the NeXT operating system's GUI back in the early ’90s.
At the time, the "X" close box was unique to NeXT, before Windows copied it. The broken X in the "Tim's home page" window means that the document has been edited and not yet saved. (A "dirty" flag). As a convenience, pressing Command/Shift/S would save back all modified web pages.
— Tim Berners-Lee, WorldWideWeb browser