For maximum compliance with W3C standards, web designers should use either the XHTML Transitional DOCTYPE or the HTML5 DOCTYPE. In truth, you can use any DOCTYPE you want, but you may experience non-critical validation errors on occasion, especially with Internet Explorer (all versions).
The DOCTYPE declaration for XHTML Transitional is:
<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN”
The DOCTYPE declaration for HTML5 is: