HTML 4.01 has three document types: strict, transitional, and frameset. XHTML 1.1 has one document type, but XHTML 1.0 has three document types, like HTML 4.01. Only one document type definition (DTD) appears in the HTML document
DOCTYPE, short for DTD, defines an HTML or XHTML document's building blocks and tells the browsers and validators which version of HTML or XHTML your document is using.
The DOCTYPE declaration must appear at the beginning of every web page document before the html element to ensure your markup and CSS are standards compliant, and that browsers handle the pages based on the appropriate DTDs.
HTML 4.01 Strict DTD:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
HTML 4.01 Transitional DTD:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd">
HTML 4.01 Frameset DTD:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"
" http://www.w3.org/TR/1999/REC-html401-19991224/frameset.dtd">
XHTML 1.0 Strict DTD:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
XHTML 1.0 Transitional DTD:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
XHTML 1.0 Frameset DTD:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
XHTML 1.1 DTD:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
Here's a basic page with the XHTML 1.1 DTD and the required head, body, and html tags.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html>
<head>
<title>XHTML Document Type Definition (DTD) </title>
</head>
<body>
<p> XHTML requires DOCTYPE, otherwise the pages won't validate and the browsers fall back on quirks mode.</p>
</body>
</html>
