Validation Tools

Why should I validate my HTML pages? is a question I hear quite often. From the WC3 Markup Validation Service:

"One of the important maxims of computer programming is: Be conservative in what you produce; be liberal in what you accept.
Browsers follow the second half of this maxim by accepting Web pages and trying to display them even if they're not legal HTML. Usually this means that the browser will try to make educated guesses about what you probably meant. The problem is that different browsers (or even different versions of the same browser) will make different guesses about the same illegal construct; worse, if your HTML is really pathological, the browser could get hopelessly confused and produce a mangled mess, or even crash.
That's why you want to follow the first half of the maxim by making sure your pages are legal HTML. The best way to do that is by running your documents through one or more HTML validators."

Help and FAQ for the Markup Validator A quick intro and help to get you started, with answers to Frequently Asked Questions. Read this first. This document has a lot of information on how the validator works, answers to frequent questions about "why did the validator say that...", as well as pointers to technical and policy documents.

WC3 Markup Validation Service

You can validate your page by typing in the url, or uploading the file from your local pc, or copying and pasting the code directly. Checking the different options will give you a different list. If you get errors, go through and fix them one by one until your page passes validation.

Screenshot of WC3 Validation Service.
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Explanation of the options.

W3C CSS Validation Service

Validates the style sheet for your site by typing in the url of the site or the style sheet. You can also validate by file upload or copying and pasting directly. You will either receive a list of errors/warnings or Congratulations, no errors found. If you have errors on the sheet, go through and correct them one by one. Revalidate until it passes.

Screenshot CSS Validation service.
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Common Validation Problems

Some of the most frequent errors the validation service finds is

Page Valet

Page Valet is another markup validator for your use.

Semantic Data Extractor

The Semantic Data Extractor  Sees a Web page from a semantic point of view. Extracts such information as outline, description, languages used, etc.

Complete List of Web Accessibility Evaluation Tools

Complete List of Web Accessibility Evaluation Tools - Web accessibility evaluation tools are software programs or online services that help determine if a Web site meets accessibility guidelines. While Web accessibility evaluation tools can significantly reduce the time and effort to evaluate Web sites, no tool can automatically determine the accessibility of Web sites.