main tag in html CSS

Question

I do find it difficult to grasp the tag.

The project I'm looking at has no opening tags, which confuses me in the first place, and only one closing tag at the end. In CSS, it also has a main. I've experimented a little with it to attempt to figure out what it does.

What the main.... in the CSS refers to is unclear to me because if I comment this out, the styling of the entire page would be messed up. As I comment out the in HTML, nothing changes at all, which I understand if it has merely an informative character.