To include an external CSS file in an HTML document, you can use the `<link>` element. This element should be placed in the HTML document's `<head>` section. The `<link>` element should have a `rel` attribute with a value of `stylesheet` to indicate that the linked file is a stylesheet and a `href` attribute with the URL of the external CSS file.

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**Advantages:**
- External CSS files can be cached by the browser, which means the browser doesn't have to download the stylesheet every time the user visits a page on your website. This can improve the performance of your website;
- It allows you to separate the styling information from the content of your HTML document;
- These files can be shared among multiple HTML documents.

**Disadvantages:**
- This can slow down the loading time of your website, especially if you have many external CSS files;
- Another potential drawback is that keeping track of all the external CSS files you're using on your website can be challenging. 

CSS, or Cascading Style Sheets, is a stylesheet language used to describe the presentation of a document written in a markup language like HTML. CSS is a fundamental building block of the modern web and controls the styling and layout of websites and web applications.

This section will cover the basics of CSS. You will learn the language's syntax, the most common CSS properties, and how to add CSS styles to your web page.

Selecting the elements you need becomes more challenging as your HTML code's complexity grows. This section will cover more advanced techniques of the selection.

This section will cover the general styling used for layout and text formatting to create a visually appealing and user-friendly design for your webpage. Part I.

Including CSS: External Styles