Validating HTML Code
W3C’s HTML Markup Validation Service is probably the easiest and the most popular tool available to check and validate your web page. Using this tool, you can spot almost every validation error ranging from missing ALT attributes for your IMG tags to placing a block-level element inside an inline element.
You can grade your HTML code by giving the address of your web page, uploading the HTML file or pasting the HTML code directly.
Validating CSS Styles
Just like the HTML validation tool, W3C’s CSS Validation Service is the most popular CSS validation tool in the web. And like the HTML validator, you can validate your CSS by pasting the URI, uploading a CSS file or pasting your code directly.
Note that this tool will detect “CSS hacks” as errors and in order to stay valid, you need to find another way to support older browsers.
Validating WordPress Feeds
Although it’s not very popular, “feed validation” is one of the another important aspect of validation of your website. And you can check your WordPress feed against errors with W3C’s Feed Validation Service for Atom and RSS. Luckily (and naturally) the default feeds of WordPress validates perfectly, so your feeds are probably already valid if you’re not using a plugin to tamper with feeds.
Additionally, there’s a noteworthy tool called Unicorn by W3C, which is basically a unified validator. Although it looks like a discontinued open-source application, it still gives satisfying results about HTML, CSS, feed and i18n validation. The results page is a bit confusing but if you ever need to get results of multiple validation tests, you’ll love this tool.
It’s A Wrap
Validating your website is essential, no question about that. As long as you validate your web pages and maintain them to stay valid, you will only benefit from it.