247 has improved the specificity for `:root` selector, but actually that was not following the correct specificity.

section selector has "0-0-1" specificity.
Original :root selector has "0-1-0" specificity, win against section.
A current replaced selector section:not(a) has "0-0-2" specificty. It will win against the simple section selector, but may lose against the complex selector.

For example, this CSS + HTML document will render text in green.

:root p { /* 0-1-1*/
  color: green;
}

html div p { /* 0-0-3 */
  color: red;
}

<html>
<body>
  <div>
    <p>green</p>
  </div>
</body>
</html>

In Marpit, this Markdown is having exactly same shape of above, but renders text in wrong color red.

<style>
:root p { /* 0-0-3*/
  color: green;
}

section div p { /* 0-0-3 */
  color: red;
}
</style>

<div>

green

</div>

:root { /* 0-1-0 */ }
section:not(a) { /* 0-0-2:  */ )
:where(section):not([\20 root]) { /* 0-1-0:  */ )

:not([" root"]) pseudo-class will always match because HTML does not allow U+0020 SPACE in the attribute name.

marp-team / marpit