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).
Links in this document showing the symbol represent key resources from The World Wide Web Consortium (W3C), especially links to the Web Accessibility Initiative (WAI) pages. W3C and WAI are the organizations that develop and maintain standards for HTML coding, Web accessibility, and related matters. Web pages that meet the WAI requirements are allowed to show the symbol, with links to what this means.