Un tip rápido o una lista que puede ser útil, una pequeña lista con los Doctypes autorizados por la W3C:

HTML 4 Estricto

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"https://www.w3.org/TR/html4/strict.dtd">

xHTML 1 Estricto

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"https://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

xHTML Transicional

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"https://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

xHTML 1.1

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"https://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

HTML 5

<!DOCTYPE html>