<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xmlns:f="http://java.sun.com/jsf/core" xmlns:h="http://java.sun.com/jsf/html" ... > </html>if we can write much simpler <!DOCTYPE html>?
<!DOCTYPE html> <html xmlns="http://www.w3.org/1999/xhtml" xmlns:f="http://java.sun.com/jsf/core" xmlns:h="http://java.sun.com/jsf/html" ... > </html>Using HTML5 DOCTYPE doesn't have side effects with Facelets parser and works fine in all browsers expect of IE6 which will render pages in quirks mode. IE6 usage is decreasing last years and it's not supported by many JSF libraries (like PrimeFaces) anymore. So, it doesn't hurt. If you are using HTML5 specific elements (e.g. file upload in PrimeFaces) and the browser doesn't support them, fallbacks will be applied. HTML5 specific elements will be gracefully degraded to HTML4.
One small issue I had during my changes of DOCTYPE was the entity which I sometimes use for non-breaking space. The Facelets parser says
The entity "nbsp" was referenced, but not declared.The solution is to replace by the character reference   Also other entities like © should be replaced by character references. Apart from that JSF components which I use in my web apps are okey so far.
Summary: Using of <!DOCTYPE html> in Facelets is highly recommended. It's simple and future-proof.