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).
But the most important META tag you should include on your web documents is the character set, or charset. This is important for the security of your web pages. In HTML 4.01, you would write: <meta http-equiv=content-type content=text/html; charset=utf-8>.