The structure of the website of a given document is very similar to its physical counterpart. In web design, however, HTML is used to structure the website. Additionally, HTML uses tags to further guide the structure of a website; they usually have a pair and are denoted by angle brackets.
Besides structure, you may want to customize the appearance of your website, which can be done using CSS. It can help to understand the website as a set of boxes that can be customized. CSS uses a selector — to denote which part of the HTML will be modified — and a declaration — to indicate the style of the HTML.
There are multiple ways you can incorporate CSS into your website. Firstly, you can do so internally, by placing the CSS in a style tag within the head portion. You can also do so externally by linking a separate CSS page with all of the rules in it.