The rel='stylesheet' attribute is used to define the relationship between the linked file and the current HTML document.

The rel stands for "relationship", and is probably one of the key features of the <link> element — the value denotes how the item being linked to is related to the containing current document.

The current HTML document needs to tell the browser what you were linking to using the rel tag, otherwise the browser would have no idea what to do with the content you're linking to.