It is possible to play Pokémon Emerald online without a download and this site www.vizzed.com/vizzedboard/retro/game.php?id=10643&syste seems to be your best option. In order to play the game on this site you need to download the Vizzed RGR Plug-in which you can do so from this link www.vizzed.com/vizzedboard/thread.php?id=8676, it should not take more than two minutes to download and is a safe site. The Vizzed RGR Plug-in is required to play N64, GBA, Genesis and 32X games. It's also recommended for SNES and also can be used, but not required, for GB, GBC and NES. The Vizzed RGR Plug-in is best used in Firefox but works in most other Internet browsers as well. If you plan to use it with Internet Explorer, you'll have to either disable IE Protected Mode or use a PC Controller. You don’t have to register to use the site but if you decide not to you will be unable to save any of your games so it may be worth it. This site lets you play all the other Pokémon games that have featured on a Nintendo GBA. Another site where you can play you can play Pokémon Emerald and other game boy games: gbemul.com/genre/action There is no need for you to make any downloads, take out any subscriptions or even register to play the game and it gives you a simple choice of all the Pokémon games that have been released on the game boy and should keep you entertained for hours on end. It is also possible to play Pokémon Emerald on your computer using an emulator and a Rom image of the game. However, the legality of this procedure is questioned. You are allowed to have a Rom of a game only if you own the original cartridge. See www.gameboy-advance-roms.com/gba_emulator.htm for more details.
