The W3C, long reverred for it’s work to develop and encourage use of standards on the web has three great online code testers — one validates that your CSS checks out OK, another tests your HTML, and the newest tester checks for problems that cause known issues on Mobile Phones.
Warning — the mobile tester is ruthless; it’s not an easy test to pass, but it’s worth the effort.
To get started, just enter your URL (or anyone else’s) into one of these online testing services at the W3C. I recommend you run all three tests on all your sites — CSS, HTML, and the mobile test site. They will quickly help you identify any common mistakes you’re making, and help you get them cleaned up. Your pages will work better in Web browsers once they are error free, and search engines will like them better, too.
Test the HTML/XHTML on your site
http://validator.w3.org/
Test how mobile friendly your site is
http://validator.w3.org/mobile/
Test the CSS (Cascading Style Sheets) on your site for mobile
http://jigsaw.w3.org/css-validator/
