image-rendering (caniuse) is a CSS Image 3 property that tells the browser how to best upscale an image. It is very useful for keeping server-side image output like that from Module:Graph crisp and readable.
The standard prescribes these names: auto | smooth | high-quality | crisp-edges | pixelated. Of these, crisp-edges and pixelated are the two most useful options we need, although you may also want to allow -webkit-optimize-contrast because Chrome (which the majority of visitors use) wants to use its own name for crisp-edges.