I'm a beginner in web development so I was reading MDN Web Docs and Chromium source code in order to understand how Javascript DOM HTML element classes work and are implemented and I found out that HTML comment elements are actually interfaces/classes (i don't know why in MDN it's called an interface if it has a constructor) (https://developer.mozilla.org/en-US/docs/Web/API/Comment). Why is it made that way? Isn't it a waste of space?
Unlike in many languages, HTML comments are part of the mark-up. You can dynamically generate them and insert them into the DOM, just like you can do with any other node.
This can potentially be useful, for example, by libraries which modify the DOM but wish to preserve comments for readability purposes. Some frameworks actually use comments to help them keep track of different nodes or lists of nodes.
Here's an example of how you might dynamically generate a comment within the HTML of a webpage. if you run these two lines in any webpage, you'll find a new <!-- Hello World! --> comment at the bottom of the body element:
const myComment = new Comment('Hello World!')
document.body.appendChild(myComment)
The ability to preserve comments actually isn't unique to HTML, other markup-parsing libraries will provide you with this kind of ability, precisely for the purposes of preserving them as you dynamically modify the markup. For example, see this yaml parser, which will automatically preserve the comments for you as it parses.