XHTML Header

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

HTML5 Header

<!DOCTYPE html>

Most current browsers do not fully support HTML5, so some sources do not recommend using the HTML5 header.
You can use either of these headers for your projects. Place the header only in the layout page.