[pavlov]

Trying to literally answer your question of who came up with this:

I'm pretty sure Windows 95 already allowed rich text on the clipboard (using the RTF format), and at least Word and Excel both supported it. Mac OS X also had RTF support from the start.

Nowadays most desktop applications write HTML to the clipboard instead of RTF, I believe. And that's how the formatting ends up in your Gmail editor inside a browser.

[compiler-guy]

MacOS Classic 6.0 (maybe even earlier) also included formatting on the clipboard. Don’t think it was rtf in those days though. But definitely preserved formatting as early as 1990. Maybe earlier.