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).
The purpose of a Document Type Definition or DTD is to define the structure of a document encoded in XML (eXtended Markup Language). For introductory material about XML, see the XML help page. It is possible to build and use files containing XML tags without ever defining what tags are legal.