body {
  font-family: sans-serif;
}

h1 {
  counter-reset: h2 h3;
}

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

h2 {
  counter-reset: h3;
}

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