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 purpose of an XML Schema is to define the legal building blocks of an XML document: 1  the elements and attributes that can appear in a document. 2  the number of (and order of) child elements. 3  data types for elements and attributes.  default and fixed values for elements and attributes.