getMatchedCSSRules with invalid Psuedo Elements On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". PASS getMatchedCSSRules(document.querySelector('p'), '')[0].cssText is "p { color: green; }" PASS getMatchedCSSRules(document.querySelector('p'), undefined)[0].cssText is "p { color: green; }" PASS getMatchedCSSRules(document.querySelector('p'), null)[0].cssText is "p { color: green; }" PASS getMatchedCSSRules(document.querySelector('p'))[0].cssText is "p { color: green; }" PASS getMatchedCSSRules(document.querySelector('p'), 'before') is null PASS getMatchedCSSRules(document.querySelector('p'), 'foo') is null PASS getMatchedCSSRules(document.querySelector('div'), 'first-letter')[0].cssText is "div::first-letter { color: red; }" PASS getMatchedCSSRules(document.querySelector('div'), ':first-letter')[0].cssText is "div::first-letter { color: red; }" PASS getMatchedCSSRules(document.querySelector('div'), '::first-letter')[0].cssText is "div::first-letter { color: red; }" PASS successfullyParsed is true TEST COMPLETE Test for https://bugs.webkit.org/show_bug.cgi?id=83298 First letter of this sentence should be red.