Validation as a debugging tool
While contemporary Web browsers do an increasingly good job of parsing even the worst HTML “tag soup”, some errors are not always caught gracefully. Very often, different software on different platforms will not handle errors in a similar fashion, making it extremely difficult to apply style or layout consistently.
Using standard, interoperable markup and stylesheets, on the other hand, offers a much greater chance of having one's page handled consistently across platforms and user-agents. Indeed, most developers creating rich Web applications know that reliable scripting needs the document to be parsed by User-Agents without any unexpected error, and will make sure that their markup and CSS is validated before creating a rich interactive layer.
When surveyed, a large majority of Web professionals will state that validation errors is the first thing they will check whenever they run into a Web styling or scripting bug.
Validation as a future-proof quality check
Checking that a page “displays fine” in several contemporary browsers may be a reasonable insurance that the page will “work” today, but it does not guarantee that it will work tomorrow.
In the past, many authors who relied on the quirks of Netscape 1.1 suddenly found their pages appeared totally blank in Netscape 2.0. Whilst Internet Explorer initially set out to be bug-compatible with Netscape, it too has moved towards standards compliance in later releases.
Validation is one of the simplest ways to check whether a page is built in accordance with Web standards, and provides one of the most reliable guarantee that future Web platforms will handle it as designed.
Validation eases maintenance
It is reasonable to consider that standards such as HTML and CSS are a form of “coding style” which is globally agreed upon. Creating Web pages or applications according to a widely accepted coding style makes them easier to maintain, even if the maintenance and evolution is performed by someone else.
Validation is a sign of professionalism
As of today, there is little or no certification for Web professionals, and only few universities teach Web technologies, leaving most Web-smiths to learn by themselves, with varied success. Seasoned, able professionals will take pride in creating Web content using semantic and well-formed markup, separation of style and content, etc. Validation can then be used as a quick check to determine whether the code is the clean work of a seasoned HTML author, or quickly hacked-together tag soup.
Frequently asked questions
Validation, as any process of debugging code, is sometimes difficult, and the vast improvements in automatic error correction has made modern browser cope very well with errors in HTML or CSS. This makes validation seem useless or costly to many people, and the following questions (or statement) are widespread:
“My site looks right and works fine - isn't that enough?”
The answer to this one is that markup languages are no more than data formats. So a website doesn't look like anything at all! It only takes on a visual appearance when it is presented by your browser.
In practice, different browsers can and do display the same page very differently. This is deliberate, and doesn't imply any kind of browser bug. A term sometimes used for this is WYSINWOG - What You See Is Not What Others Get (unless by coincidence). It is indeed one of the principal strengths of the web, that (for example) a visually impaired user can select very large print or text-to-speech without a publisher having to go to the trouble and expense of preparing a separate edition.
“Lots of websites out there don't validate - including household-name companies.”
Do remember: household-name companies expect people to visit because of the name and in spite of dreadful websites. Can you afford that luxury?
Even if you can, do you want to risk being on the wrong side of a lawsuit if your site proves inaccessible to - for instance - a disabled person who cannot use a 'conventional' browser? Accessibility is the law in many countries. Whilst validation doesn't guarantee accessibility (there is no substitute for common sense), it should be an important component of exercising "due diligence". It is now just over a year since a court first awarded damages to a blind user against the owners of a website he found inaccessible (Maguire vs SOCOG, August 2000).