Details

Pseudo-elements, like ::before, and ::first-letter, should be declared with two colons. Pseudo-classes, like :hover and :first-child, should be declared with one colon.

If you're sure the pseudo-element in question is valid, submit a request to add it to the default whitelist.

Bad: wrong colons

p:before {
  content: '>'
}

p::hover {
  color: red;
}

Good: correct colons

p::before {
  content: '>'
}

p:hover {
  color: red;
}