Create pages that use the transitional or the strict document type declaration and include the appropriate declaration as the first line of every HTML page.
There are three Doctypes usable with XHTML or HTML documents: Strict, Transitional, and Frameset. Each Doctype describes in precise, computer-readable language, the allowed syntax and grammar of XHTML markup.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
Extensibility : Under HTML, the addition of a new group of elements requires alteration of the entire DTD. XML greatly eases the integration of new element collections as it is a subset of SGML itself and specifies it's own DTD.
Portability : By the year 2002 as much as 75% of Internet access could be carried out on non-PC platforms such as palm computers, televisions, fridges, automobiles, telephones, etc. In most cases these devices will not have the computing power of a desktop computer, and will not be designed to accommodate ill-formed HTML as do current browsers.
XHTML DTD - W3Schools defines three Document Type Definitions.
HTML version information - The global structure of an HTML document.
DOCTYPE - Document Type Declaration.