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).
Where no existing HTML element is applicable, span and div can valuably represent parts of a document so that HTML attributes such as class, id, lang, or dir can be applied. span represents an inline portion of a document, for example words within a sentence.