By default, Visualforce pages are served with a doctype of HTML 4.01 Transitional. Specifically, pages begin with this doctype declaration:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
You can specify a different doctype for a Visualforce page by using the docType attribute on the <apex:page> tag.
The docType attribute takes a string representing the document type. The format of the string is:
<doctype>-<version>[-<variant>]
where
If an invalid document type is specified, the default doctype is used. For more information about valid HTML doctypes, see the list at the W3C website.
To create a Visualforce page with an XHTML 1.0 Strict document type, use the docType attribute on the <apex:page> tag, and specify a value of xhtml-1.0-strict:
<apex:page docType="xhtml-1.0-strict" title="Strictly XHTML" showHeader="false" sidebar="false"> <h1>This is Strict XHTML!</h1> <p> Remember to close your tags correctly:<br/> <apex:image url="/img/icon-person.gif" alt="Person icon"/> </p> </apex:page>