Yep, my guess would be that they are using a markdown parser that generates a complete html document out of submitted markdown, and then they just stitch those parsed htmls together, like here (the processed_html method on the article instance):

https://github.com/thepracticaldev/dev.to/blob/master/app/vi...

> Browsers are very resistent to garbage markup

Miraculously so! Anything I would have attempted to design would have crapped its pants at the look of this markup, but browsers heroically manage to display it as if nothing weird is happening. I take my hat off to their resilience.