the app is a API for allow create apps with HTML, in the official site have a example of app.
API stands for application programming interface—a set of definitions and protocols to build and integrate application software.
class... just class...
paste this in the body
<script src="https://app-html.esdrasde2.repl.co" type="module">
!type="module"
for await and async functions
paste this in the head
<link rel="stylesheet" href="https://app-html.esdrasde2.repl.co">
<