HTML , by virtue of its SGML heritage, is a language for describing the structure of a document, not its actual presentation. The idea here is that most documents have common elements—for example, titles, paragraphs, and lists. Before you start writing, therefore, you can identify and define the set of elements in that document and name them appropriately
If you’ve worked with word processing programs that use style sheets (such as Microsoft Word) or paragraph catalogs (such as FrameMaker), you’ve done something similar; each section of text conforms to one of a set of styles that are predefined before you start working.
HTML defines a set of common elements for web pages: headings, paragraphs, lists, and tables. It also defines character formats such as boldface and code examples. These elements and formats are indicated inside HTML documents using tags .