How to add CSS

When a browser reads a style sheet, it will format the HTML document according to 
the information in the style sheet.
There are three ways of inserting a style sheet:
External CSS
Internal CSS
Inline CSS
With an
external style sheet, you can change the look of an entire website by changing 
just one file!
Each HTML page must include a reference to the external style sheet file inside 
the <link> element, inside the head section.
External styles are defined within the <link> element, inside the <head> section of an HTML page:
An external style sheet can be written in any text editor, and must be saved with a .css extension.
The external .css file should not contain any HTML tags.
Here is how the "mystyle.css" file looks:
Note: Do not add a space between the property value and the unit:
  Incorrect (space): margin-left: 20 px;
  Correct (nospace): margin-left: 20px;
An internal style sheet may be used if one single HTML page has a unique style.
The internal style is defined inside the <style> element, inside the head 
section.
Internal styles are defined within the <style> element, inside the <head> section of an HTML page:
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.
Inline styles are defined within the "style" attribute of the relevant 
  element:
Tip: An inline style loses many of the advantages of a style sheet (by mixing
content with presentation). Use this method sparingly.
If some properties have been defined for the same selector (element) in different style sheets,
the value from the last read style sheet will be used. 
 Assume that an external style sheet has the following style for the <h1> element:
Then, assume that an internal style sheet also has the following style for the <h1> element:
If the internal style is defined after the link to the external style sheet, the <h1> elements will be 
"orange":
However, if the internal style is defined before the link to the external style sheet, the <h1> elements will be 
"navy": 
What style will be used when there is more than one style specified for an HTML element?
All the styles in a page 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

Inline style (inside an HTML element)
External and internal style sheets (in the head section)
Browser default
So, an inline style has the highest priority, and will override external and 
internal styles and browser defaults.
Try it Yourself »
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Add an external style sheet with the URL: "mystyle.css".
<head>

</head>

<body>
  <h1>This is a heading</h1>
  <p>This is a paragraph</p>
  <p>This is a paragraph</p>
</body>

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