Much is missing from the speficiation (SVG standard), like the definition of spot-light cone anti-aliasing. My first idea was similar as how FireFox works (based on the hints in the specification), but it was neither looked good, nor matched for the expected values. Finding more expected values, I did some reverse engineering, and now I can mimic the SVG expected values.
Created attachment 56688 [details] proposed fix
Created attachment 56689 [details] proposed fix
Created attachment 56694 [details] patch without filters-light-01-f-expected.png svn-apply does not yet support git binray delta diffs. I removed the filters-light-01-f-expected.png, to allow to run the bots.
Comment on attachment 56694 [details] patch without filters-light-01-f-expected.png r+ to get the W3C expected results.
Patch landed in r60466: http://trac.webkit.org/changeset/60466 As we discussed I sent questions about the anti-alias of the cone to the www-svg mailing list, but no reply so far.