The following page exhibits some artifacting (looks tile-related?): http://jsfiddle.net/48ts9q2o/show/light/ The scales on the page are ridiculous which is probably the root issue. This bug occurs in both Safari 8.0.7 and WebKit nightly at r187013. Edge, Chromium, and Firefox get this right (although each has their own special perf issue :)
Seems to work in Safari for me.