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W3C-style background gradients have spaces stripped from within the property value by the CSS filter. This makes them invalid.
background: linear-gradient(to bottom,#fff8ba 0%,#eccf90 100%);
Investigating this report has turned up that it's the cssutils-based filter and not the rCSSMin-based minifier that's causing this.
It looks like merging #934 would likely solve this.
#934 is a discussion pull request and not intended to be merged at this time. But yes, it would solve the issue (among other issues).
Fixed in dce6b6d.