SDL (Simple DirectMedia Layer) is a cross-platform, open-source C library that gives low-level access to graphics, input, and audio hardware. It’s used by a huge range of video playback software, emulators, and classic game source ports — including Chocolate Keen, the source port I used for my WebAssembly Commander Keen port.
Since Emscripten has SDL support built in, getting the game’s original SDL rendering calls working in a browser tab was mostly a matter of compiling against Emscripten’s SDL implementation rather than rewriting the graphics layer from scratch.