.document {
  counter-reset: h2;
}

.document h2 {
  counter-reset: h3;
}

.document h3 {
  counter-reset: h4;
}

.document :not(#table-of-contents) h2::before {
  counter-increment: h2;
  content: counter(h2) ". ";
}

.document :not(#table-of-contents) h3::before {
  counter-increment: h3;
  counter-reset: h4;
  content: counter(h2) "." counter(h3) ". ";
}

.document h4::before {
  counter-increment: h4;
  content: counter(h2) "." counter(h3) "." counter(h4) ". ";
}