Most often, the purpose of data validation is to ensure correct user input.
Furthermore, some browsers are less forgiving of bad HTML than other browsers like Internet Explorer (which can be very forgiving).
Buy Now Mobile devices have become very popular, and many people may be viewing your website on a mobile device.
Any HTML or CSS problems could be especially harmful for viewers using browsers on mobile devices, so this is another great reason to validate your web documents.
Different browsers use different rendering engines so tricks used to make HTML render as desired in one browser may not work in another browser or even in an earlier or later version of the same browser.
Fortunately, browsers today are driving toward standards compliance, and proprietary elements like "layer" and "ilayer" are being dropped.
Search for validating web:
This might include Firefox, Google Chrome, Opera, Safari, Internet Explorer and the increasing number of mobile browsers.