Wouldn’t it be great if you could look at the web page you are developing and be able to instantly tell if something is amiss. Say you forgot to add alt tags to your images, or you are using deprecated markup.

This might be a case for Holmes, the CSS Markup Detective.

What Holmes does is add CSS styling to elements in an HTML page that do not conform to standard, and will highlight any markup that is non-compliant, deprecated, or missing required attributes. All you need to do to make it work is to include a single CSS file in your web page, and you can then see the results immediately.

Since Holmes uses the W3C HTML5 standard (as it stands now), it is particularly useful for modernising old websites or existing codebases without too much trouble.