This is really weird! My experiments have been executed on v1.0.179, and the CSS works there as might be expected.
On v1.0.190 however, the greater-than sign in the selectors part make the entire rule invalid. Even one > operator is fatal. I have no explanation for this. The rules that follow (coloring the H5 and H6) still work, so the <style>...</style> block has not completely been blown up.
This concerns only the CSS specs in the note itself. When I copy them to userstyle.css, they work again as expected. So there must some extra intermediary interpretation of style blocks embedded in notes that is malfunctioning now. New issue!