Diablo 1 is playable in your browser

The original Diablo is now playable in your browser thanks to the efforts of  GalaXyHaXz and the devilution team. The project was made possible after Diablo's entire source code was leaked online from Japan (Climax Studios was tasked with porting the game on PlayStation but due to negligence on Sony's part the source code was leaked online). They have released their work on GitHub were they have also talked about the project:

Symbolic information was accidentally left on the Japanese port. Normally used for debugging, a symbol file contains a map of everything generated during compile time. This includes file names, functions, structures, variables, and more! To top it all off a special build is hidden on the PC release in DIABDAT.MPQ -> D1221A.MPQ -> DIABLO.EXE! This build contains debug tools and assert strings further giving away code information.

From this leaked info the team started working on Project Devilution which now made Diablo playable in your browser. The devs have also explained the purpose of the project. The wanted to preserve the game in its original form, so they even kept they bugs and other badly written code:

Having the source code makes Diablo much easier to update and maintain. For years mod-makers had to rely on tedious code editing and memory injection. A few even went further and reversed most or all of the game. The problem is that they rarely shared their work. Usually being a one-person job, they move on with their lives due to the amount of time required or lack of interest. This brings us back to square one having to do countless hours of work all over again. Devilution aims to fix this by finally making the source code open to the community.

In order to ensure that everything is preserved, Devilution keeps everything as it was originally designed. This goes as far as bugs and badly written code in the original game. With that it serves as a base for developers to work with making it much easier than before to update, fix, and port the game to other platforms.

The version available for all is a Shareware version, so that means players aren't able to do much in-game (you can only play as a warrior, and are free to explore the first two levels of the dungeon), but you can also unlock the full game by uploading your original DIABDAT.MPQ file to the site.

Diablo 1 + the Hellfire Expansion are both available on GOG for 9 eur or your regional equivelent, so obtaining the DIABDAT.MPQ file is still possible.