I remember that the original Windows 8 Mail client has some issues with media queries, specifically a webkit-based declaration work-around needed to ensure the responsive code wasn't being shown first. Has 8.1's mail client changed anything? Thanks!
I just ran a test, and it appears that you no longer need to use (-webkit-min-device-pixel-ratio: 1) with Windows 8.1's mail client.
Ben, that's awesome to know - that extra expression was a little annoying for a few customers :)
Just to show, it pays to have a Surface Pro afterall :)