There is no reason your website should not conform to W3C standards. It will ensure the highest possible ability, of your website, to be displayed properly in the wide variety of web browsers available in the marketplace. Clearly there is above and beyond, specifically with regard to semantics and accessibility, but W3C validation should be considered the bare minimum.

If your spending a fair amount of time creating a website, you want it to display properly. If 60% of your visitors are on Internet explorer, 20% on Firefox, and 15% on Opera, and 5% on other you want to make sure your website displays fine on everything including the 5% other. This can be tricky sometimes, and even if your website is W3C compliance you may have to dive into scemantics to get it to display properly in the 5% other.

If you have a W3C validated website then you have relatively clean code that you can attack scemantically to improve the accessibility in the 5% other. Start by validating your website. If your website is straight html, or xhtml code then you must edit your .html file using a text editor. Using a graphical editor the code may result in W3C compliant code, but it will be easy, most likely, to improve things scemantically.

Ultimately you want your code to be easy to read and investigate. Take a peak at how my code is written on WaterFuelConverters. Hit view in your browser and look at the source. You want your website to be broken down with divs, and you want to predominantly use external CSS and javascript. If you need an inline javascript to call a particular script then seeing it in the rendered code is understandable. Also if your a little lazy a little inline CSS is understandable. By externalizing CSS and javascript it will be easier to view the structure of your website, particularly the content.

Make sure you have alts on images, and titles on links. This help with SEO and accessibility. You can also use detective javascript to render different code depending on the browser. This is a last resort though because it hinders accessibility. If your website is being rendered in a browser without javascript then it the targeted code will not be rendered accordingly. Using javascript to render tergeted code is useful for the vast majority of visitors; particularly Internet Explorer and Firefox. Most of the time these two browsers render things comparably, but sometimes with the older versions of IE things get funny. There are still many users that have IE 7, and suprising people still be IE 6 and earlier. You’ll want to make sure your browser render fine in, at least, Firefox, IE 7 & 8, Opera, and Safari. With these browsers vieweing your website fine, you are covering the vast majority of viewers.