<style>
div.ol {
    counter-reset: index;
    padding-left: 1em;
}
div.li {
    counter-increment: index;
}
div.li::before {
    content: counters(index, ".") ": ";
}
</style>

<div class="ol">
    <div class="li">A</div>
    <div class="li">B</div>
    <div class="li">C</div>
    <div class="li">
        <div class="ol">
            <div class="li">D</div>
            <div class="li">E</div>
            <div class="li">
                <div class="ol">
                    <div class="li">F</div>
                    <div class="li">G</div>
                    <div class="li">H</div>
                </div>
            </div>
            <div class="li">I</div>
            <div class="li">J</div>
        </div>
    </div>
    <div class="li">K</div>
    <div class="li">L</div>
    <div class="li">M</div>
</div>