A CSS filter that references a filter defined in SVG, like "filter: url(#foopy);" always uses sRGB interpolation. It should use linearRGB if the SVG does not explicitly choose sRGB. This is what other browsers do.
Created attachment 339633 [details] Test case
Created attachment 339756 [details] Patch
https://trac.webkit.org/changeset/231473/webkit
<rdar://problem/40045449>