Code snippet: Automatic Header Numbering Pure CSS

body {
    counter-reset: section;
}

h2 {
    counter-reset: subsection;
}

h3 {
    counter-reset: subsubsection;
}

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

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

h4:before {
    counter-increment: subsubsection;
    content: counter(section) "." counter(subsection) "." counter(subsubsection) " ";
}


<h1>Title</h1>
<h2>Content-Section 1</h2>
<h3>Subsection 1</h3>
<h3>Subsection 2</h3>
<h4>SubSubsection 1</h4>
<h2>Content-Section 1</h2>
<h3>Subsection 1</h3>
<h4>SubSubsection 1</h4>
<h4>SubSubsection 2</h4>
<h3>Subsection 2</h3>
<h3>Subsection 3</h3>
<h4>SubSubsection 1</h4>


Title
1. Content-Section 1
1.1 Subsection 1
1.2 Subsection 2
1.2.1 SubSubsection 1
2. Content-Section 1
2.1 Subsection 1
2.1.1 SubSubsection 1
2.1.2 SubSubsection 2
2.2 Subsection 2
2.3 Subsection 3
2.3.1 SubSubsection 1