I think the history goes: agar.io and slither.io were surprise smash-hit games that both featured simple, drop-in-and-play multiplayer HTML5 with a dedicated .

I think the history goes: agar.io and slither.io were surprise smash-hit games that both featured simple, drop-in-and-play multiplayer HTML5 with a dedicated .io domains. They inspired literally hundreds of copycats that each set up .io domains to signal that they were following they genre. The genre has become so popular that "io game" is now a term.