I find problems in Gecko much less often than I find problems in Blink. For example, WebKit handles border-collapse by drawing borders on top of one another, which is totally bogus when transparent borders are involved. While not a spec violation per se, Blink's behavior when absolute containing blocks are split across multiple lines is really bizarre (objects parented to that containing block just disappear). WebKit violates the HTML spec when the top margin of an element at the top of the document is queried in quirks mode. Etc. etc.