HTML and XHTML Doctypes have been around for a long time, but in order to make Web that bit easier for novice webmasters, who don't always understand intricacies of Doctypes, it will soon be time for all of current Doctypes to bow-wow out gracefully and make way for something new:
Dogtype Declarations are easy to use. You simply choose Dogtype that most closely matches characteristics of your website.
Here, from newly-formed 'Woof Woof Woof Consortium' is a handy beginners' guide to available Dogtypes:
Collie or Sheepdog Dogtype - Use this DTD if your site is there to do a job, rather than being there merely for entertainment purposes. Suitable sites will be totally professional, ultra-reliable, and never let you down, although they might be a trifle dull or lacking in surprises at times.
Poodle Dogtype - for Flash-heavy sites that are all style and very little substance. Why poodle? Think dyed pink and over-pampered showdogs, usually called Fifi and often seen sporting lots of bows or ribbons and other canine bling.
Whippet or Greyhound - For fast-loading sites. Not too many images. No animated gifs. Lightweight stylesheet.
Mutt or Mongrel - For sites that have been around since dawn of time. The site owners have only just heard about Doctype declarations and decided to stick first doctype that comes along at top of their page. Anything's better than Quirks mode, they think. Ha! But if backwards compatibility is an issue for you then use Mutt/Mongrel. Labrador - A useful Dogtype if you need to address issues of accessibility particularly with regard to blind users.
Pit Bull - Suitable for any site that grabs your browser, locks on and simply refuses to let go, so you can neither click to go back, forwards or anywhere and have to do a Control-Alt-Delete manoeuvre in order to escape. This usually happens because of images that have not been properly optimised. Or there may be too many animations. Or scripts. On rare occasions these problems might not actually be caused by website. It could just be that your old Commodore 64 just can't handle today's fast-paced Internet. But usually it is site that is at fault.