Prerequisites for template inheritance?

Are there prerequisites for template inheritance? I ask because it does not appear to work in VSC. I'm familiar with Symfony\Twig and the use of {% extend "templates/base.html.twig %}. Yet I am unable to reproduce the effect in VSC. For example:

{% extends "base.html" %}
{% block body %}
booga wooga
{% endblock %}

is rendered as {% extends "base.html" %} {% block body %} booga wooga {% endblock %} even though base.html contains

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="utf-8">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <title>Castle Rock HOA</title>
        <link rel="stylesheet" href="assets/css/bootstrap.min.css">
    </head>
    <body>
        {% block body %}
        {% endblock body %}
    </body>
</html>

ecmel / vscode-html-css

Prerequisites for template inheritance? #260