style.css

body { counter-reset: h1counter h2counter h3counter h4counter h5counter h6counter; }

h1 { counter-reset: h2counter; }
h2 { counter-reset: h3counter; }
h3 { counter-reset: h4counter; }
h4 { counter-reset: h5counter; }
h5 { counter-reset: h6counter; }
h6 {}

h2:before {
    counter-increment: h2counter;
    content: counter(h2counter) "\0000a0";
}

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

h4:before {
    counter-increment: h4counter;
    content: counter(h2counter) "." counter(h3counter) "." counter(h4counter) "\0000a0";
}

h5:before {
    counter-increment: h5counter;
    content: counter(h2counter) "." counter(h3counter) "." counter(h4counter) "." counter(h5counter) "\0000a0";
}

h6:before {
    counter-increment: h6counter;
    content: counter(h2counter) "." counter(h3counter) "." counter(h4counter) "." counter(h5counter) "." counter(h6counter) "\0000a0";
}