Tutorial :What DOCTYPE should I target today?


I'm refactoring a .Net web application that is in

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >   

Right now the approach is just to aim for the stars and go for the latest doctype just because it's latest, I would like to make a wiser choice and target a specific one and for good reasons.

There are similar questions existing but the answers might be outdated now.

What is the difference, advantages, disadvantages between standards and quirks mode, what are some quirks I may run into with differently set doctypes?

I have been told that an XHTML doctype is preferable to integrate AJAX since the UpadtePanel serializes it and to do so needs to have a XHTML do type, to what extent is this true?

And for browser compatibility, in which direction are browsers going in terms of DOCTYPE, is there a common thrend or do they differ?


HTML5 doctype, which is

<!DOCTYPE html>  

XHTML is largely dead as a standard, and never was implemented correctly in most cases.


Any Doctype:

  • HTML 4.01 or XHTML 1.0
  • Strict or Transitional

served as html (not html+xml) should be OK. There's no such thing as a better doctype, you just have to choose one filling your needs and then stick to its rules.

  • Avoid Frameset, but if you've to, use the title attribute to describe the role of each frame to a screen reader user (same with iframe btw).
  • Quirks mode (no Doctype) is a PITA, avoid it at all cost. This was OK 8 years ago.
  • No XML prologue unless you're serving html+xml (good luck with that! If you like complicated things when it's not needed, that's your choice)
  • If you are forced to use attributes that are forbidden in Strict mode (target="_blank" for example) than use Transitional mode: this is why it was created! And please indicate to your users that the link will open in a new page, whether in the text of your link or in its title. This is important from an accessibility point of view.

HTML 5 is the next big thing, we're waiting for it but as long as it won't work in every browser (I mean IE without JS) it's not advisable to use it in "serious" public sites. Is it even a Draft? What if entire part of it are rewritten in a couple of months?
My web agency uses it for its website but we won't use it on a client site anytime soon: it's just too soon.

Sidenote: I often see catch phrases like "a modern website in HTML5 and CSS3" implying that CSS3 is made for HTML 5. CSS3 has nothing to do with HTML5 and can already be used, as long as it degrades gracefully on old browsers.
You can design HTML5 with CSS2.1 or HTML4.01 Transitional with the latest CSS3 animations that only work in webkit nightlies, no problem.


The new thing is HTML 5.

<!DOCTYPE html> is what you use to specify it. That's it. No DTD name or URL or whatever.

If you're using something that likes XML, like .net, then you might want to use XHTML. But don't do it for any other reason; XHTML never was really popular as a standard, or at least it was almost never used correctly.


Whatever you choose, make sure your MIME-Type is compatible with your DOCTYPE

The browser will use the MIME-Type (the HTTP Header ContentType) to determine how to treat your page. For example: A DOCTYPE of XHTML 1.1 Strict served as ContentType Text\HTML is parsed as HTML.

DOCTYPE is important, but largely irrelevant if the wrong ContentType is used.


Browsers have never actually used DOCTYPE to determine the markup language of your document (they use HTTP Content-type instead), so which DOCTYPE you chose was never hugely relevant - just as long as you are using a valid DOCTYPE of some description. Whichever you choose is up to you.

If you're writing HTML, <!DOCTYPE html> is the shortest to type, and puts all browsers into standards mode (which is what you want).

If you're writing XHTML, <!DOCTYPE html> is also perfectly legitimate (XHTML actually requires no DOCTYPE at all, as it relies entirely on HTTP Content-type, but there's no harm putting a DOCTYPE in for portability.

Don't use <!doctype html> - while this is technically valid HTML, it's invalid XHTML so will break if you ever try to parse your page as XML.

Slightly OT sidenote: Some people here have commented that XHTML is a "dead" standard - this is false. XHTML has been integrated into the upcoming HTML5 spec. The spec is entitled "HTML5: A vocabulary and associated APIs for HTML and XHTML"


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