Just stumbled across this comprehensive MS Beta web page test tool:

http://www.modern.ie/en-us

What's this tool about? Today it can take a lot of time to make your web experiences look great (or just work) across various browsers and devices. This tool detects common coding practices that may cause compatibility problems or prevent your users from getting the best possible experience on a webpage. Whenever possible we suggest a fix or enhancement with web standards like HTML5 & CSS3 (or a graceful fallback). It's not a complete checklist for coding the modern web just a starting point so you can spend less time testing for IE and more time building what matters on the web.

This scanner is BETA (build 75 2-April 2013). Send feedback to help us improve modern.IE.
I like the Responsive Test Results:
We've found that this webpage is using responsive web design practices to support many browsing devices. This helps your users get the better site experience on devices like Apple iPad, Microsoft Surface, Amazon Kindle, XBOX 360, and other modern browsers on smaller or larger screens.

Breakpoints should be defined according to the content on your site. We suggest the following common breakpoints as a starting point to target desktop, tablet, and mobile form-factors:

The graph below shows the window widths for which we detected media query support (in green). We recommend testing your site at the non-highlighted window widths to confirm it works well.
NOTE: This graph uses common breakpoints which may help you target specific form-factors.