Stack Snippets are runnable JS/CSS/HTML codeblocks within the questions/answers that have recently been implemented on StackOverflow.
I don't know as there are enough HTML5 game development questions to warrant the feature, but this question brought the feature to mind, and I thought it might be something useful to have. Especially to mitigate dead links to fiddles or pens in those instances.
It might also be useful for explaining or showing certain algorithms; e.g. how to layer octaves of noise for PCG and a resulting visualization. Or for GLSL/OpenGL questions that can be implemented within the subset of WebGL.