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Why specify a DOCTYPE? Why validate?


From: Mark Pilgrim
Date: May 30, 2002 6:31AM

First off, let me disclaim the question by saying that I maintain a site of several hundred pages which all validate as XHTML 1.1 and are all Section 508 compliant. (They would be Bobby AAA compliant too, if Bobby were a little less buggy, but that's a different topic.) I write about accessibility from time to time, and I practice what I preach.


I have never figured out why specifying a DOCTYPE or having markup that could pass validator.w3.org is important to accessibility.

WAI 1.0 says only this:
"Validating to a published formal grammar and declaring that validation at the beginning of a document lets the user know that the structure of the document is sound. It also lets the user agent know where to look for semantics if it needs to."

WAI 2.0 (working draft, April 24, 2002) says this:
"Checkpoint 5.1 [4.2] Use technologies according to specification.

You will have successfully met Checkpoint 5.1 at the Minimum Level if:

except where the site has documented that a specification was violated for backward compatibility, the markup has passed validity tests of the language (whether it be conforming to a schema, DTD, or other tests described in the specification)."

...All of which boils down to "your documents should validate because we say so," which is unsatisfying.

I understand that specifying a DOCTYPE is important because you can't validate without one, and validation is important because validation is important, but why is validation important for accessibility? Are there user agents that validate a document at rendering time, or that require valid documents? Screen readers read broken documents just fine; Lynx displays broken documents just fine; invalidity doesn't interfere with text zoom or screen magnifiers or any other assistive technology I can think of.

Furthermore, a nested table-based layout and no header tags could be a perfectly valid document and still be an inaccessible mess, contradicting WAI 1.0's claim that validity implies any measure of structural soundness.

So why specify a DOCTYPE? Why validate?

Please CC, I'm on digest.

-Mark Pilgrim

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