How does Xenforo apply max-width to images?

[Amin Sabet]

I noticed that my Xenforo forum will automatically scale max width of images to fit the browser window in IE, Chrome, etc. In other forum software, setting image max-width to 100% works for Chrome but not IE.

I've never been able to get images on websites to respect a max-width based on browser size in IE. Can someone tell me how Xenforo accomplishes this?