A warning IS graceful degradation, an error or unexpected behaviour would be ungraceful. The channel that warnings come over is separate from the channel the content is being rendered in and it shows, for those that look at it, that the system is degraded and an action would need to be done to restore it to a non-degraded state.
This is not about graceful degradation. An HTML document without any CSS should never be in a degraded state. It should be perfectly usable and perfectly well accepted.
I agree; a HTML document without CSS should be perfectly usable and OK (and ideally CSS should never be required; if CSS is disabled it ought to still work OK, too).
However, if CSS specifies some things and omits others that are related to it (one example is specifying the font size for one heading level but not another one; another example is specifying the background colour without specifying the foreground colour or vice-versa), then it makes sense to be a warning.
I can't articulate it and I could very well be on the wrong vibe, but this feels like the bad practice of relying on the failure mode within a try/catch for normal functioning.
