HTML Validator is an extension to, well, validate HTML code based on Tidy. After installation, it displays an icon on the lower right corner of your browser which will tell you if the page you are viewing has any errors in it.

Fig 8. Page with no error or warning


Fig 9. Page with some errors and warnings

If a page has no error or warning, you will see a green icon with a checkmark on it. Otherwise it turns into a red round icon (Fig 9) or a yellow triangle (not shown here). Double-click on the icon and you will be presented with the view-source page, only this time with extra information at the bottom (see Fig 10).

Fig 10. Source code with error and warning messages

The top window displays the source code as usual with the lines that produce error/warning highlighted. The bottom-left windows has a list of errors and warnings. Click on any line and you will be presented with the culprit line and the explanation on the bottom-right window. Nifty, eh?