They're using Eleventy and Netlify, those are not HTML either; Eleventy implies they start with markdown too I think?
I'll forgive them image formats and the .js file for Twitter. But, they follow up with a heap of "lets use tables for design" examples, always hated that ... I'm nope-ing pretty hard there.
I started writing HTML in the 90s using Pico, my roots are purist, but no, not really ... you need styles for a11y, you want styles for user comfort and engagement, even minimal things like favicons and svg. We've come a long way, yes, bloated messes like Sharepoint spews out are abysmal, but to make a website that meets any reasonable standards ... you need more than just HTML in this dinosaur's opinion.