For the following piece of code from ua.css: > div:-moz-native-anonymous.moz-accessiblecaret.right > #image { > } > > div:-moz-native-anonymous.moz-accessiblecaret.none { > } > > @media (min-resolution: 1.5dppx) { > div:-moz-native-anonymous.moz-accessiblecaret.normal > #image { > } > } Stylo reports > Unknown pseudo-class or pseudo-element ‘-moz-native-anonymous’. Ruleset ignored due to bad selector. for the two rules outside @media, but it reports just > Declaration dropped. for the rule inside the @media. This causes browser/base/content/test/static/browser_parsable_css.js to fail because "Declaration dropped." is not in the whitelist, and it shouldn't either.