Use at your own risk. This is only useful if you want fluid retina images to work in Gmail and Outlook without using VML fallbacks.

<img src="/path/to/image-600x600.jpg" width="300" height="" height="300" border="0" style="display: block; width: 100%; height: auto; max-width: 300px; max-height: 300px;" />

Outlook happily reads the second height attribute, while every other browser/client reads the first. Images with a blank height attribute behave the same as height: auto, which is necessary for fluid images. This is totally invalid markup since you can't have duplicate attributes. But that doesn't bother you, right?