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).
At the very top of the page you will see the doctype declaration: <!DOCTYPE html> * Like any language, HTML5 has a grammar and a vocabulary. Grammar <!DOCTYPE html> goes at the top of every HTML5 page. Vocabulary The HTML5 word <!DOCTYPE html> means this page is written in HTML5 as opposed to, say HTML 4.01.