<!DOCTYPE> Declaration
A valid HTML or XHTML document is required to tell the browser which version of the markup language has been used in the writing of the
document and this is done using the <!DOCTYPE> element. The element contains a document type declaration giving the Document Type Definition (DTD)
used.
The different DTD's relate to which elements can or cannot be used in the document and are as follows.
HTML Document Type Definitions
HTML 4.01 Strict - Documents given this DTD contain all elements and attributes apart from those which are deprecated and those that are used in frameset documents.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http:/www.w3.org/TR/html4/strict.dtd">
HTML 4.01 Transitional - This DTD conains all HTML elements including deprecated elements but not frames.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http:/www.w3.org/TR/html4/loose.dtd">
HTML 4.01 Frameset - This DTD contains everything in the Transitional DTD and frames are also included.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"
"http:/www.w3.org/TR/html4/frameset.dtd">
XHTML Document Type Definitions
XHTML 1.0 Strict - May contain all elements and attributes apart from those which are deprecated and those which relate to frames. As the DTD relates to XHTML the markup must be well-formed as explained here XHTML vs. HTML.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http:/www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
XHTML 1.0 Transitional - May contain all elements and attributes including deprecated but excluding frameset elements. XHTML markup must be well-formed.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http:/www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
XHTML 1.0 Frameset - This DTD may contain all elements and attributes including deprecated and frame/frameset elements. As with the other XHTML DTD's the markup must be well-formed.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"http:/www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
Attributes
NONE