HTML tags define both the structure of a web page and the way the marked text displays in a browser such as Netscape Navigator.
Tags mark the start and the end of text.
For example, you can use the H1 tag to mark text as a first-level heading. When a web browser such as Netscape Navigator displays that text, it appears in a large font, HTML documents may have different appearances on different browsers. Even if all of your readers use Netscape Navigator, they all might not see your HTML page in the same way because they can configure Netscape Navigator to use different fonts and to override your color settings.