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).
Whether you want to parse HTML files via an SGML or an XML parser, or simply make sure that your document gets parsed the same way across HTML5 processors, the doctype is a critical component of a quality HTML document. Further Reading. Fixing your site with the right Doctype, an article on A List apart by Jeffrey Zeldman