Hi
Some many time, while developing web based project, It will look proper on one browser,but it doesn’t look proper on other browser. I had faced this problem so many time.
Here is some free tools which will help to check the XHTML and CSS standards.
If we don’t follow this standard then it will not look proper on all browser.
To validate HTML and XHTML
http://validator.w3.org/
To validate CSS
http://jigsaw.w3.org/css-validator/
Chandradev's Blog 
I am not certain the place you’re getting your information, but good topic.
I needs to spend some time learning more or figuring
out more. Thank you for fantastic info I was searching for this info for my mission.