There are two "filter" CSS properties in WebKit:
1. "filter" - parsed for SVG filter
2. "-webkit-filter" - parsed for the CSS filter used in HTML content
The problem is that the value of one is not going to affect the other. From WebKit's point of view they are completely different CSS properties.
Some frameworks like prefixfree are detecting that "-webkit-filter" can be unprefixed just becaue there is a "filter" property available. However, that will fail because the SVG filter is not used in HTML and also has just a subset of the CSS Syntax.
Link to the original bug in prefixfree: https://github.com/LeaVerou/prefixfree/issues/76
Eventually -webkit-filter will get a subset of filter and vice versa.
This was resolved a long time ago.