::before and ::after pseudo-elements

I’ve noticed that Chrome does not render the ::before and ::after pseudo-elements. The spec doesn’t appear to specify whether they should or shouldn’t be supported. I would expect that most CSS features would be supported unless explicitly specified not to.

My use case was trying to implement something like the old <menclose notation="downdiagonalstrike"> with CSS:

<mrow class="cancel">
  <mn>1</mn><mo>+</mo><mn>1</mn>
</mrow>

.cancel {
  position: relative;
}

.cancel::after {
  background: linear-gradient(
    to bottom right,
    transparent calc(50% - 0.1ex),
    currentColor calc(50% - 0.05ex),
    currentColor calc(50% + 0.05ex),
    transparent calc(50% + 0.1ex)
  );
  content: "";
  inset: 0;
  position: absolute;
}

I could off course just add the background to the original element, but if the line is a different color than the content, then the line is drawn behind the content.

Start by reading the issue discussion and the linked HTML and CSS examples, focusing on whether ::before and ::after should be supported for the shown MathML structure. Done means the specification clearly resolves the support question and records the expected behavior for this use case.