It's not reporting errors just inefficiencies. It's suggesting your CSS is more complicated than it needs to be. I'd recommend examining the suggestions, especially those listed as "very inefficient". There's nothing technically wrong with 2 or 3 descendant selectors (form table .item-list ul) but it makes things hard to maintain and modify, and if there are lots of them it will slow things down and can really be a problem in large projects. If you strive and learn to keep things simpler, minimal and terse you'll be better off, a better coder.