A new feature in Firefox (follow link below) allows you to see why certain CSS rules are not having an effect.

Why isn’t this CSS doing anything?
There are a number of property & value combinations that can lead to CSS being inactive, and now Firefox will tell you why. Open the developer tools, and look for the greyed-out property with an info-box on hover.

This is super useful, but would've been even more useful when I was just starting out with web development. The kids these days just don‘t know how good they‘ve got it!