Edited since comments below

Something that I've come across recently is that the Windows 8 mail app (Metro) doesn't appear to support background-size when declaring it as its own property.

I often use this CSS property when optimising images for HiDPI devices by serving images at 2x, and then using background-size to scale them down.

<td align="left" valign="top" class="logo">
    <a href="http://google.co.uk" target="_blank">
        <img src="image-1x.jpg" alt="A logo" style="display:block;" width="205" height="57" border="0" />
    </a>
</td>

This fails in the mail app:

@media (-webkit-min-device-pixel-ratio: 1.5), (min-resolution: 144dpi)  { 
    td[class="logo"] img { display:none !important; }
    td[class="logo"] a {
        display:block; 
        width:205px; 
        height:57px;
        background-image:url(image-2x.jpg); 
        background-repeat:no-repeat; 
        background-size:205px auto;
    }
}

However using the shorthand background property works OK.

@media (-webkit-min-device-pixel-ratio: 1.5), (min-resolution: 144dpi)  { 
    td[class="logo"] img { display:none !important; }
    td[class="logo"] a {
        display:block; 
        width:205px; 
        height:57px;
        background:url(image-2x.jpg) 0 0 / 205px auto no-repeat;
    }
}

Meanwhile both variants work OK in all other CSS3 capable email clients I've tried. I've observed this behaviour in Windows 8/8.1 and Windows 10 (Technicial Preview).

The whole HiDPI image swapping stuff also doesn't work so well in Outlook.com due to the lack of background-image support, but you can workaround this with the specific .ExternalClass CSS class to overrule any image replacement attempts.

Outlook.com CSS support taken from Campaign Monitor - https://www.campaignmonitor.com/css/

The rendering engine of the mail app is Internet Explorer, so this is a quite a strange behaviour difference because IE9 and above has CSS3 support, and background-size support has never been an issue in the newer versions.