How much affect search results if the web is written with valid code - i mean without bugs or so? Is there any difference? Any handicap to PR when the web isn´t valid?
I think it would really depend on how bad the bugs are. Major bugs could prevent search engine bots from properly crawling and indexing your website. Smaller bugs probably have no affect. Forgetting to close one tag isn't the end of the world. It is still in your best interests to validate all your html. This ensures that all visitors to your website see your website as intended. I wouldn't be surprised if search engines did penalize sites with too many errors in their code.
Most sites today are not full valid. I really don't see this as being an issue unless you have some sort of bug that is preventing spider access to the site.
If you have the chance to make it 100% valid - why not? Maybe SEs don't pay attention to it now, but they might in the future... So it's better to do it now and have less worries later.
though some scripts are crawlable they are not indexable, but when theres problem with page loading bcoz of this bugs, this might affect your rankings...
I have validated my sites and fixed "most" of the code... I have also validated some of the top contenders in Yahoo and Google and feel that super messy code wont keep you out of the top 3. Could be like Vansterdam said... May really depend on what tag is open, etc, or severity of the invalid code... I'd rather forget a "<\b>" somewhere than a ">" at the end of my description meta tag.
I cleaned up the code on one of my homepages the other day and it helped me jump a couple of places for my main keyword. This could have been coincidence though....
My homepage is valid, but it does not need to be valid. There are a lot of other pages which are not valid, but popular. So it is not so important.
And what does PR have to do with it? You can get PR9 for a site with ugliest code you can imagine, that doesn't mean anything. We're talking about SERPs