table-of-contents.liquid

{% assign heading_level = "2" %}
{% assign opening_tag = "<h" | append: heading_level %}
{% assign closing_tag = "</h" | append: heading_level | append: ">" %}
{% assign toc = content | split: opening_tag %}

<nav class="page-toc">
  <ol>
  {% for item in toc %}
  {% if item contains closing_tag %}
    {% assign tag = item | split: closing_tag | first | strip %}
    {% assign attrs = tag | split: ">" | first | strip %}
    {% assign id = attrs | split: "id=" | last | split: " " | first | remove: "'" | remove: '"' | strip %}
    {% assign title = tag | split: ">" | last | strip %}
    <li><a href="#{{ id }}">{{ title }}</a></li>
  {% endif %}
  {% endfor %}
  </ol>
</nav>