Test your site on different platforms.
Each browser has different style of reading codes,especially theres huge differences between mozilla firefox and internet explorer,sometimes mozilla firefox can predict wrong codes but internet explorer is not enough clever to do what firefox does even though microsoft engineers made it.( the only thing they can do well is copying apple mac OSs anyway)
The web sites linked below will test your web site ,it will show how speed is your web site on mobile phones,on firefox,ie and other browsers and some of the sites will show your code errors etc.
Webxact shows your web site’s load time,total bytes of your web site and broken links on your site,but the problem is webxact scans very slowly,instead of webxact you can use pingdom to see your website’s detailed information.See my blog’s information from pingdom ;
Clean Css scan your web site’s css and converts long codes to shorter ones such as 0px to 0,#FFFFFF to #FFF and it shows errors in CSS.After fast scanning my CSS became like this : Input: 73.617KB, Output: 66.708KB, Compression Ratio: 9.4% (-6909 Bytes)
Its a famous css validator which shows your web sites css errors.
Review your website on different mobile phones like nokia,motorola,sony ericson,sharp,see load time of your website when connected via Wifi,3G and GPRS,it will also show the cost of loading your web site.
This tool downloads your website and it shows you tips about what should you do to make your web site better.