You can use the built in and plugin web developer tools for each major web browser. These are constantly being updated but all share common functionality for debugging HTML, CSS, Javascript and network activity.
Highly popular web development tool. But it is easy to get dependent on this tool and forget to test in all other web browsers. There are lite versions for Safari and Chrome to supplement their included tools.
The IE test tool is free. It runs in Windows allows you to have the rendering and javascript engines of IE10, IE9, IE8, IE7 IE 6 and IE5.5 on Windows 8 desktop, Windows 7, Vista and XP, as well as the installed IE in the same process. It is an alpha version and thus has a higher potential for crashes.
Web developer IE toolbar, DOM inspector , view http headers, javascript debugger, css inspector.