Most people claim you get higher rankings if your HTML code, Cascading Style Sheets (CSS) and RSS-feed are W3C compliant. I don’t always agree; it’s perfectly possible to rank a site without having every small detail just the way the World Wide Web Consortium specifies.
Of course there are limitations. If you make too much mistakes in your HTML, CSS or feed, your rankings will be affected, but some small mistakes will be neglected (but you should always at least try to produce code as clean as possible). This is not the case for your RSS-feed. If it doesn’t comply, it might not be useable in every feedreader and you might lose a lot of visitors. So there are times you might want to check the validation for your (or someone else’s) website.
Checking the validation of your HTML, CSS and feed with the W3C validation services is not that hard, but with the shortcuts I am offering you, it’s really easy.