Get a css property from a DOM-element selected by given selector. The return value
is formatted to be testable. Colors gets parsed via rgb2hex
and all other properties get parsed via css-value.
Note that shorthand CSS properties (e.g. background, font, border, margin, padding, list-style, outline,
pause, cue) are not returned, in accordance with the DOM CSS2 specification - you should directly access
the longhand properties (e.g. background-color) to access the desired values.