Emulation in Flash, JavaScript etc.

If I were to play games on a website such as ssega.com or snesfun.com, what format would the games be on? For example, would the games be considered normal ROMS, or would they be considered Public Domain ROMS? Or maybe ISOs? Just wanted to know, I probably sound stupid for posting this.

Playing games on line could be any number platforms. Some online games are real js emulators playing roms on the server, others are clones.