In biology, cell theory is a scientific theory which describes the properties of cells. These cells are the basic unit of structure in all organisms and also the basic unit of reproduction.
The doctype declaration should be the very first thing in an HTML document, before the <html> tag. The doctype declaration is not an HTML tag; it is an instruction to the web browser about what version of the markup language the page is written in. The doctype declaration refers to a Document Type Definition (DTD).
When you save an HTML document, you can use either the .html or the .htm file extension. HTML consists of the set of markup symbols or codes inserted in a file intended for display on a World Wide Web browser page. The markup tells the browser how to display a Web page's words and images for the user. HTM and HTML are the most common type of documents used to build static webpages. Both HTM and HTML may contain links to images, audio, video and scripts.