To play Pokémon Emerald online, one must have an emulator of the original game. Various sites offer emulators and also feature other retro games available for play.
Keep in mind that emulators, ROMS and any other copies of licensed video game software are typically illegal; Nintendo specifically discusses this issue at their legal site. Despite the stated illegality of such software, they are still readily available online to play in browsers and as downloads. There is a lot of debate swirling around online about the gray areas of the use of such software, but be aware that to possess downloads of such software could leave one open to a civil charge of piracy.