External Stylesheet Use

Use the link attribute in the document’s head:

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

You can also use stylesheets provided from websites via a content delivery network, or CDN for short. (for example, Bootstrap):

<head>
    <link rel="stylesheet" href="<https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css>" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
</head>

Generally, you can find CDN support for a framework on its website.