First question I got, it asked for a "valid CSS selector" with a certain specificity.
I tried for a while. I couldn't figure what was wrong. There was a fairly long and obtuse parsing error below the input box. I tried different selectors all with the wanted specificity.
It turns out that I didn't have to write just the selector; it needed a full rule, even if empty.
The thing is this...
#someId p.class
...is a valid CSS selector. Meanwhile, this...
#someId p.class { }
...is a CSS rule (or sometimes ruleset). You can see https://developer.mozilla.org/en-US/docs/Web/CSS/Syntax for a more visual explanation.