Hypertext Markup Language (HTML) is the standard markup language for documents that are designed to be displayed in a web browser. HTML is not a programming language. It is a markup language that tells browser how to structure a web page. HTML is made up of sereis of elements such as a piece of text, a pragraph, list, table, links or forms.
As in the above example, html document consists of a tree of elements and text. Each element starts with a tag and ends with a tag. Tags have to be nested such that they are completely within each other without overlapping .
Block-level elements form a visible block on a page. A block-level element appears on a new line following the content that precedes it. Any content that follows a block-level element also appears on a new line. Block-level elements are usually structural elements on the page. For example, a block-level element might represent headings, paragraphs, lists, navigation menus, or footers. A block-level element wouldn't be nested inside an inline element, but it might be nested inside another block-level element.