We're all about helping your do your job better. Here are some useful tools and resources we think are helpful in designing and developing websites.
W3C Markup Validation
The W3C (World Wide Web Consortium) sets standards for web technologies such as HTML, CSS, XML, etc. They also provide a "code validator" that allows you to ensure your website code conforms to standards.
To validate your code enter the URL (the address...like www.mywebsite.com) for the webpage you'd like to validate.