When I analysed the CSS style sheets of some SE websites, I ran across some typos.
Stack Overflow (also meta):
.suggested-edit .body-diffs .full-html-diff .gutter /* Selector */ max-width:10x; /* Property */ ^ Replace `x` with `px`
.answered-accepted .mini-counts AND .answered-accepted strong borer-bottom:1px solid #f2fcea; ^ Replace `borer-bottom` with `border-bottom`
I encountered these errors when I parsed the CSS using code beautifier, to check the possibility of overriding the TeX' stylesheet using Stylish, to be more comfortable. If you find other errors in other SE sites, feel free to post them as an answer.
PS. Do not add an answer about
\9, valid CSS3 or vendor-specific properties, as the parser is like a human: Capable of committing mistakes.