Besides the box-shadow, transform and border options mentioned in other answers, WebKit browsers currently also obey -webkit-text-fill-color to set the colour of the "time elapsed" numbers, but since there is no way to set their background (which might vary with platform, e.g. inverted high-contrast modes on some operating systems), you would be advised to set -webkit-text-fill-color to the value "initial" if you've used it elsewhere and the audio element is inheriting this, otherwise some users might find those numbers unreadable.