Doctypes for XHTML docs
For XHTML strict: (Use this doctype when you're not using any deprecated or frameset tags with XHTML.)
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
For XHTML transitional: (Use this doctype if your document is in XHTML and contains deprecated tags.)
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
For XHTML frameset: (Use this doctype if your document is in XHTML and contains frameset tags and/or deprecated tags.)
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
Doctypes for HTML docs
For HTML strict: (Use this doctype if your document is in HTML and contains no deprecated tags.)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
For HTML transitional (or loose): (Use this doctype if your document is in HTML but contains one or more deprecated tags.)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
For HTML frameset: (Use this doctype if your document is in HTML and contains frameset tags and/or deprecated tags.)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">