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).
[RFC3688] defines an IANA maintained registry of XML documents used within IETF protocols. These documents are divided into classes: XML Namespaces, Document Type Declarations, Schemas, and RDF Schemas. The following are the entries for the 'publicid' class of documents.