css without separate file
snippet in html

html style sheet

user6111

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

external style sheet

user499

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

how to refer to external style sheet

user4931

<html>
<head>
<link rel="stylesheet" type="text/css" href="Name.css">
</head>
<body>
.
.
.
</body>

how to link external css to html

user9282


  <!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="mystyle.css">
</head>
<body>

<h1>This is a heading</h1>

  <p>This is a paragraph.</p>

</body>
</html> 

css without separate file

user4921

just add any style inside the first <p> or other element tag using style=" ;" 
<p style="font-size: xx-large;">test</p>