SSIA.

We already have this partially but add two more rules:

/* <mrow>-like elements */
semantics > :not(:first-child) {
  display: none;
}

maction > :not(:first-child) {
  display: none;
}
merror {
  border: 1px solid red;
  background-color: lightYellow;
}
mphantom {
  visibility: hidden;
}

___

We already have last two so it is just re-arranging and adding first two.

Thanks!