lang/style/counters.css

:root {
  --section-prefix: '';
}

h1::before, h2::before, h3::before, h4::before, h5::before, h6::before {
  padding-right: 1ex;
}

main h1 {
  counter-increment: h1;
  counter-reset: h2 h3 h4 h5 h6;
}

main h1::before {
  content: var(--section-prefix) counter(h1);
}

main h2 {
  counter-increment: h2;
  counter-reset: h3 h4 h5 h6;
}

main h2::before {
  content: var(--section-prefix) counter(h1) '.' counter(h2);
}

main h3 {
  counter-increment: h3;
  counter-reset: h4 h5 h6;
}

main h3::before {
  content: var(--section-prefix) counter(h1) '.' counter(h2) '.' counter(h3);
}

main h4 {
  counter-increment: h4;
  counter-reset: h5 h6;
}

main h4::before {
  content: var(--section-prefix)
    counter(h1) '.' counter(h2) '.' counter(h3) '.' counter(h4);
}

main h5 {
  counter-increment: h5;
  counter-reset: h6;
}

main h5::before {
  content: var(--section-prefix)
    counter(h1) '.' counter(h2) '.' counter(h3) '.' counter(h4) '.'
    counter(h5);
}

main h6 {
  counter-increment: h6;
}

main h6::before {
  content: var(--section-prefix)
    counter(h1) '.' counter(h2) '.' counter(h3) '.' counter(h4) '.'
    counter(h5) '.' counter(h6);
}