height:auto Hack for Fluid Retina Images in Gmail
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?