<link href="main.css" rel="stylesheet">This simple example provides the path to the stylesheet inside an href attribute, and a rel attribute with a value of stylesheet. The rel stands for „relationship“, and is one of the key features of the <link> element — the value denotes how the item being linked to is related to the containing document.
The ralationship with the document, това е ключовата дума. Демек, какво е – CSS, икона, JS…
In HTML, link types indicate the relationship between two documents, in which one links to the other using an < a >, < area >, < form >, or < link > element.
Литература:
https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link#attr-rel