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).
Consists of markup code that indicates the grammar rules, or Document Type Definition (DTD), for the particular class of document. The document type declaration can also point to an external file that contains all or part of the DTD. It must appear following the XML declaration and preceding the document element.