The W3C allows you to validate your HTML documents and CSS style sheets using their HTML validator (validator.w3.org) and CSS Validator (jigsaw.w3.org/css-validator/). When validating your page's codes and styles, you should first validate its HTML, since invalid HTML can interfere with validating your page's CSS.
To validate your document's HTML, your document must include a valid DocType declaration that declares which version of HTML is being validated (the transitional version of HTML 4.01, for example). It also must include a META element declaring the document's character set (see "Declaring Your Document's Character Set" earlier in this session). To validate a local HTML document, access the W3C's HTML Validator, click the Browse button (next to the Local File field), navigate to and select the HTML file you want to validate, and then click the Validate File button.
To validate your style sheet, first highlight and copy your style sheet's styles in your text editor. Next, access the W3C's CSS Validator, scroll down and click on the "with a text area" link, and then paste your styles into the provided text area box. Then just click the button to validate your styles.
It is important to distinguish between errors and warnings. An error means that something in your style sheet is in violation of the CSS specification. In order to have a valid CSS style sheet, you must first correct the error condition. A warning, on the other hand, advises you of a condition that goes against good practice. You do not need to correct a warning to have a valid CSS style sheet, but doing so will help ensure that your document will be accessible to all.
Once you've validated your page's codes and styles, you can display graphic logos, provided by the W3C, that declare your page as conforming to the HTML 4.01 and CSS2 Web standards.