While validation will remain your front-line defense against pages that render poorly in the browser, there is a suite of other testing tools that can help you determine that you’ve created your page the right way, most especially for search engines. As a broad rule, these tools should be used after you’ve validated the page.
While the W3C validator will remain the canonical standard for web page validation, there are a number of other possible validation services that offer some extra features.
Validator.nu is a good alternative to the W3C validator that accepts a URL to a live site, a file upload, or direct input of code.
Both the W3C and Validator.nu validate a page at a time; W3CLove checks all the pages of a live site, which is very useful for small-to-medium-sized sites.
Climbing Mount Sinai: The Importance of Web Page Validation
A browser provided with HTML code must choose how it will display the web page. Even at the best of times browsers will tend to display web pages in slightly different ways; provided with broken code, each browser may show completely different results.
The best way to avoid this issue is to feed every browser valid code that follows the W3C specification. Any remaining variations in the way a browser displays your web page should be dealt with as exceptions (for example, feeding IE8 information wrapped in conditional comments.)