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).
The HTML <body> tag is used for declaring the main content section of the HTML document. The <body> tag is placed after the document's head (i.e. between the </head> and the </html> tags). The basic tag is written like this <body></body>, with the document's content inserted between the opening and closing tags. On any normal-sized HTML document, you'll usually see the opening tag (<body>) near the top of the document and the closing tag (</body>) near the bottom.