In Firefox (not sure about the others), they do give some info on some errors, such as what might be overriding a rule (other rules or not an appropriate displ

In Firefox (not sure about the others), they do give some info on some errors, such as what might be overriding a rule (other rules or not an appropriate display type or something) or some invalid property or value. Not perfect, but it is not all in the dark. And the ability to directly manipulate the CSS and see the impacts can make getting the right visuals fairly quick, not to mention the ability to change the view size and directly interact with that in all the different screen sizes.