:root:first-child { background-color: red; }
:root:last-child { background-color: red; }
:root:only-child { background-color: red; }
:root:nth-child(1) { background-color: red; }
:root:nth-child(n) { background-color: red; }
:root:nth-last-child(1) { background-color: red; }
:root:nth-last-child(n) { background-color: red; }
:root:first-of-type { background-color: red; }
:root:last-of-type { background-color: red; }
:root:only-of-type { background-color: red; }
:root:nth-of-type(1) { background-color: red; }
:root:nth-of-type(n) { background-color: red; }
:root:nth-last-of-type(1) { background-color: red; }
:root:nth-last-of-type(n) { background-color: red; }
p { color: green; }

<p>This line should be green (there should be no red on this page).</p>