Will making your website up to W3C Standards likely affect how your website is ranked by search engines.
That's not true at all. While making sure your page was entirely standards compliant was important about 8 months to a year ago, currently Google's algorithm has taken a lot of the weight off of standards compliance and put it into things such as page speed, freshness of content, quality of content, incoming links, and quality of incoming links. The google spiders are becoming more and more human-like with each update, and they literally can read a web page like a human could. So if the content is there, relevant, and fresh the spiders will recognize it as valuable no matter if its perfectly standards compliant or not. I'm not saying you can sacrifice organization though. Make sure you still have an organized link hierarchy and proper navigation because this is how the spiders will find their way around the site. Also, make sure you sign up for a google webmasters account so you can submit your sitemap directly to google. Once all of this is done, submit to the google index. Primarily I would focus on your page speed if you're dealing with a lot of users and freshness of content and incoming links if you're selling something/dealing with lower traffic.
Well, W3C validation does not affect SEO at all. You can find many examples on web. In google sites are itself not validated on w3c
Google does not need to crawl their own sites... big companies don't care, their clients will come anyway. If your code is valid it is always better ... most people here are not able to do valid code because they use WYSIWYG programs like DreamWeaver or Expression Web... those programs add useless code to your site... Some time ago I was reading a blog post where they said that invalid code can lead bot's to stop crawling. I always do valid (X)HTML for several reasons. Those standards were not made for fun. At the moment I am focused at Front-End Performance Optimization... all my sites I did have Google Page Speed score between 90/100 and 98/100
W3C standards not affect to SEO at all. However, Might that one part of SEO. W3C more affect to internet browser, for example border radius for make rounder corner, why border radius got error in W3C? because not all internet browser can support border radius, e.g Internet Explorer and opera. In IE or Opera, border radius still look a box.
Maybe you should inform yourself what the W3C is. It looks to me that you do not know anything about validation and/or CSS properties. border-radius is a property of CSS3 and CSS3 is NOT a standard yet. The W3C CSS validator only supports until CSS level 2.1, that's why CSS3 properties give an error.
Allow me to reiterate, if you CAN build your site w3c standards compliant then do it! Perfection never hurt anyone. But don't kill yourself over it. If you have limited time, focus your efforts on other pursuits than making it entirely w3c compliant. One great example of a site that is nowhere close to compliant is MySpace. MySpace is a huge site used by millions of people throughout the world, yet the whole thing is coded in tables and extreeeeemely sloppy unintelligible CSS for the most part, yet still manages to do ok. The main thing you should be worrying about is organization, content, page speed and usability. If a human can't figure it out, a robot won't either, no matter how clean your code is.
It's about semantic code... when you know how to code then you always do valid code automatically. It's not about limited time. All the big players like FB, Google, twitter and even crap MySpace have a big audience and do not need to care about SEO like you and I have to care about. If you want to sell quality web design... you should also sell quality code behind your design. Negligent (X)HTML code can be done by nearly everyone that is able to use Photoshop... just slice your design into a table... those so called "webdesigners" have NO clue about (X)HTML at all. Quality ... not only the quality content counts. Period. Usability goes without saying. Front-End Performance Optimizing (optimizing sites for faster loading speed) can NOT be done by everyone. Some people are still using all their CSS inline, some people still load JS in the head, some people are just thinking that a WP plugin like Super Cache is the all-in-one solution. It's definitely much more you can or have to do. Cookieless Domains, Minifying, Combining, decreasing HTTP requests, using CSS sprites, ... I did that on a page that has more JS and more content then the Google start page. I have Google Page Speed score 98/100 and an average loading time of 80ms ... Google has 95/100 and 178ms loading time...
Well if you ask me, my own blog is not coded well like CSM has already described that some people are already using JS in the head, i am one of them, but the thing is that google spiders has their priority they would first consider then quality of content and speed and later they would like to see is the web well coded or not, but as CSM said that if its not well coded spiders might not be able to see your fresh or quality content, and yah people like us should follow the standards, as if your website or blog is not according to the standards, it will for sure effect its page rank but you can't say that it will effect that much that it will throw you from page rank 6 to 0. At the end i would like to say don't put your hands into so much work that you can't handle all and make you feel that you should ignore SEO and put effort in content. take care
W3C compliance is always a good idea! Not only will it "not hurt", but it improves cross browser compatibility and SEO (no matter what is said). Semantic code is not imperative to have a website function and rank well, but it does help. And, it certainly doesn't hurt your marketability!
Site loading speed is a factor that is considered by google so wp3 standard is beneficial for your site seo.
w3c is only for cross border browser compatibility, it cannot help your seo standing. take an example of a wp page, it is difficult to get it to be w3c compliant due to the various different plugins