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 text between <html> and </html> describes the web page, and the text between <body> and </body> is the visible page content. This Document Type Declaration is for HTML5. It is the basic building blocks of all websites. The language instructs browsers to display text, objects and images to be displayed correctly.