These are HTML and CSS validators that I use regularly to ensure clean code which means websites should render well in any browser.
Pages that don't render well in certain browsers increase a site's bounce rate which is a key factor search engines take into account when ranking a site.
Pages don't have to be 100% valid, in fact many add ons like Facebook boxes can cause validation issues, but everything that can be resolved within your control should be.
There's no perfect CSS validator because it depends how you look on things. Theoretically not everyone has browsers that can handle the latest CSS3 styling. But large numbers of webmasters have decided that small detail can go hang because you can do so much more in CSS3 and anyone who doesn't have a browser that can handle it probably isn't going to be spending any money anyway!
Validators, on the other hand, try to tread a more cautious line and throw up errors if your website is using CSS3. It's important to know how to interpret them. For example is it important if a box of content doesn't have a shadow in for browsers who can't handle CSS3? Probably not. As such you need to know what you are doing in order to be confident the errors shown won't be affecting how your pages render across the vast numbers of browsers that now exist.
On a final note many webmasters get very obsessed about Validators. Louis Lazaris excellent article How to Validate CSS3 provides good general guidance about giving validators the right amount of attention ... but not too much.
I'm Tim Hill, a Search Engine Optimisation and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
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