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).
An HTML file consists of text, that is, the words that you want to appear in your document and embedded instructions called HTML tags. The tags give the browser instructions that include formatting, display of images, and hyperlinks. The technical name for the type of data in an HTML file is ASCII text.