Learn HTML:

html link tag

The <link> tag defines a link between a document and an external resource.

The <link> tag is used to link to external style sheets.

<head>
<link rel="stylesheet" type="text/css" href="theme.css">
</head>

In HTML the <link> tag has no end tag.

In XHTML the <link> tag must be properly closed.

Attribute	Value	Description
charset	char_encoding	Not supported in HTML5. Specifies the character encoding of the linked document
crossorigin	anonymous use-credentials	Specifies how the element handles cross-origin requests
href	URL	Specifies the location of the linked document
hreflang	language_code	Specifies the language of the text in the linked document
media	media_query	Specifies on what device the linked document will be displayed
rel	alternate archives author bookmark external first help icon last license next nofollow noreferrer pingback prefetch prev search sidebar stylesheet tag up	Required. Specifies the relationship between the current document and the linked document
rev	reversed relationship	Not supported in HTML5. Specifies the relationship between the linked document and the current document
sizes	HeightxWidth any	Specifies the size of the linked resource. Only for rel="icon"
target	_blank _self _top _parent frame_name	Not supported in HTML5. Specifies where the linked document is to be loaded
type	media_type	Specifies the media type of the linked document