I found this today while reading dzone.
Its interesting that someone actually went tyhrough the effor to produce a test for the search engine's preference.

So, maybe writing good code is worth it after all...

There is also a sticky on this subject:
http://www.webmaster-talk.com/html-f...-should-i.html

Interesting Paul. I'm not sure their test is all that scientific. With only 4 pages it could simply be Google happened to index that one version of the page first.

It would also depend on what validation errors they made. Some errors could make it impossible for a page to be crawled and others wouldn't make a difference.

The test also seems out only to see which version of a page would be chosen as the original version and which would be seen as duplicate. There's no correlation that the page will rank any better or worse.

agreed, its probably not the most scientific.
Its just that I see so many postings with random speculation when simple tests may be able to answer better.

Agreed. I think it was an interesting experiment and I'm sure a somewhat better test could be devised. I'm a supporter of writing valid code anyway so I don't need convincing.

I do see claims all the time that valid code will improve ranking, but I've yet to be convinced by the proof offered. I want to believe it though and true or not I'm still planning on validating my code.

Is w3school validation is must for search engine ranking?

It's not a must at all. Most sites that rank well don't validate. Now if you make some really bad mistakes in your code it could set up roadblocks to getting indexed and ranked.

Validating your site will help clean up those errors. Sites that validate do tend to display better across browsers and there are some who'll say that cleaner valid code can help in ranking. But no it's not at all a must.

Valid XHTML ensures that the page will work correctly in the future.
I don't think google puts much weight on valid code, for example:
http://validator.w3.org/check?uri=www.google.com

But i would think its somewhere in google's code, if you are in a competitive market, why not use valid XHTML, it can only do good.

-Brett

Good points Brett. It's not like valid code is going to hurt at all. There are good reasons to write valid code outside of a search engine and while I don't really see them requiring valid code any time soon you never know when they might decide to give a boost to pages that validated or start seeing it as a sign of quality.

And writing valid code isn't hard. There's a bit of a learning curve with it, but it's not hard.