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).
ISO Language Codes. The HTML lang attribute can be used to declare the language of a Web page or a portion of a Web page. This is meant to assist search engines and browsers. According to the W3C recommendation you should declare the primary language for each Web page with the lang attribute inside the <html> tag, like this: <html lang=en>.