Number headers with CSS

style.css

body {
  counter-reset: h2counter;
}

h1 {
  counter-reset: h2counter;
}

main h2:before {
  content: counter(h2counter) ".\0000a0\0000a0";
}

main h2 {
  counter-increment: h2counter;
  counter-reset: h3counter;
}

main h3:before {
  content: counter(h2counter) "." counter(h3counter) ".\0000a0\0000a0";
}

main h3 {
  counter-increment: h3counter;
}