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 <strong> tag is a phrase tag. It defines important text. Tip: This tag is not deprecated, but it is possible to achieve richer effect with CSS. All phrase tags: In HTML 4.01, the <strong> tag defines strong emphasized text, but in HTML5 it defines important text. The <strong> tag also supports the Global Attributes in HTML. The <strong> tag also supports the Event Attributes in HTML.