CSSLint expects image-rendering to be a depreciated value.

[Kikasuru]

CSSLint expects optimizeQuality or optimizeSpeed for the image-rendering property, despite those values being depreciated.

[ferdnyc]

So it turns out that comes from the https://github.com/CSSLint/parser-lib dependency, which does indeed still support only the deprecated values. I suspect that's because they're there for SVG, and in that context they're still the only valid values for the SVG image-rendering attribute (along with auto).

Another problem is that apparently no browser actually supports smooth, which is marked as experimental, except Firefox 93+. Even pixelated requires a fairly recent browser. (Then again, optimizeSpeed and optimizeQuality are also only supported by Firefox 93+.)

At least crisp-edges has much more widespread support. That should definitely be recognized, at the very least.

Still, like I said, this appears to be an issue for parser-lib rather than csslint.