Chapter 5. Testing CSS

In my experience, CSS errors make up the vast majority of problems with websites. If you follow web standards, once you've got the content on the page in a website, you'll spend most of your time trying to get it to look right in one browser or another. Unfortunately, the disparate state of browsers heightens the likelihood that you're going to have to make adjustments to your CSS. The good news is that browsers, on a whole, are moving closer together in how they render web pages. In this lesson, you learn a few techniques for uncovering issues with your site's CSS before your client does.


Before you start testing your pages in browsers, you want to make sure all of your CSS proverbial i's are dotted and t's are crossed. To assure your CSS syntax is error-free, you validate it. CSS is based on a specification, known as a recommendation, developed by the W3C. The CSS specification is used by online applications called validators to check your files for accuracy and make sure there are no unsupported selectors, properties, or values.

The most frequently used CSS validator is hosted by the W3C itself. The W3C CSS Validation Service (Figure 5-1) is located at and can check CSS in a variety of formats:

  • By URI: URI, short for Uniform Resource Identifier, is the parent term of the more frequently used URL (Uniform Resource Locator). A URI can refer to a web address or a local file path. For the CSS Validation Service, ...

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