Validation
Validation is a process of checking documents against formal standards. In addition to yielding maintenance-friendly files, valid markup and code "future proof" files as browsers become standards-compliant.
Validation Resources
Markup Validation Service
Check the markup of XHTML documents.
CSS Validation Service
Check your CSS.
HiSoftware Cynthia Says
Check Section 508 or W3C WCAG 1.0 accessibility guidelines.
WebXACT
Check accessiblity compliance. (This tool was formerly known as "Bobby.")
Link Checker
Find dead links and fix them.
Firefox Web Developer Toolbar
Lots of tools for evaluating file information and structure and for examining pages as they appear with and without CSS, Javascript, images, etc.
Evaluating Web Sites for Accessibility with the Firefox Web Developer Toolbar
Another use for the toolbar.
Firefox Javascript Console
Debug Javascript. Click the red x/green checkmark in the browser's upper right corner to display the console.
