If you want a quick way to validate your web pages, try the Firefox HTML Validator extension. Install the extension, restart Firefox and you will notice a new icon in the bottom right-hand corner of your window. When your mouse pointer hovers over that icon, Firefox displays a tooltip with the number of errors, if any, in the current page. Select that icon and Firefox will display a View Source dialog with more information about errors in that page. Then, if you select the OK button in that dialog, Firefox displays a new window with the source view of the current page in one pane and two extra panes containing errors and help.