Three Ways to Insert CSS
There are three ways of inserting a style sheet:
External style sheet
Internal style sheet
Inline style


External Style Sheet
With an external style sheet, you can change the look of an entire website by changing just one file!

Each page must include a reference to the external style sheet file inside the element. The element goes inside the section:

Example



An external style sheet can be written in any text editor. The file should not contain any html tags. The style sheet file must be saved with a .css extension.



Here is how the "mystyle.css" looks:

body {
background-color: lightblue;
}
h1 {
color: navy;
margin-left: 20px;
}

Internal Style Sheet
An internal style sheet may be used if one single page has a unique style.

Internal styles are defined within the element, inside the section of an HTML page:

Example


body {
background-color: linen;
}
h1 {
color: maroon;
margin-left: 40px;
}

Inline Styles
An inline style may be used to apply a unique style for a single element.

To use inline styles, add the style attribute to the relevant element. The style attribute can contain any CSS property.
The example below shows how to change the color and the left margin of a

element:

Example
This is a heading

Cascading Order
What style will be used when there is more than one style specified for an HTML element?

Generally speaking we can say that all the styles will "cascade" into a new "virtual" style sheet by the following rules, where number one has the highest priority:

Inline style (inside an HTML element)
External and internal style sheets (in the head section)
Browser default
So, an inline style (inside a specific HTML element) has the highest priority, which means that it will override a style defined inside the tag, or in an external style sheet, or a browser default value.