CSS:

Cascading Style Sheets is a style sheet language used for describing the presentation of a document written in a markup language such as HTML .

There are 3 types of CSS in html:
1.Inline
2.internal
3.External

INLINE:

This type is written as:

<p background colour, border 2px solid (any colour)></p>

INTERNAL:

This type is written as:

EXTERNAL:

This type is written by connecting another file with our main file:

And connected with original file using link  tag: