How does Internet Explorer 11 differ between Windows 7 and Windows 8.1?

IE11 seems to be slightly different on Windows 7 to Windows 8(.1). I guess this is because some features are tied to the OS.

E.g. the IE11 on Windows 7 developer preview blog post briefly mentions SPDY:

On Windows 8.1, IE11 also supports the SPDY network protocol

And DRM video:

On Windows 8.1, IE11 supports the latest media streaming standards, Media Source Extensions (MSE) and Encrypted Media Extensions (EME).

Are there any other differences that web developers need to be aware of (especially CSS and JS APIs)? Is it worth downloading 2 separate VMs for testing IE11?

Here are 12 things MSFT lists as unsupported in IE11 on Win7: http://msdn.microsoft.com/en-us/library/ie/dn394063(v=vs.85).aspx#unsupported_features

• Automatic phone number detection
• Device Orientation events
• Drag and drop touch support
• Effects batching and stroke improvements
• Encrypted Media Extensions
• Enhanced Protected Mode (EPM) AppContainers (see below)
• F12 developer tools UI Responsiveness (available after Win7 update)
• High DPI support
• Hover touch support
• Link highlighting
• Media Source Extensions
• Pinned site enhancements
• Screen Orientation API
• Scrolling and zooming with touch and other inputs
• Syncing across devices
• SPDY

Windows 8.1 also uses AppContainers for Isolation in Enhanced Protected Mode while Windows 7 does not support AppContainers. http://blogs.msdn.com/b/ieinternals/archive/2012/03/23/understanding-ie10-enhanced-protected-mode-network-security-addons-cookies-metro-desktop.aspx

http://blogs.msdn.com/b/ieinternals/archive/2013/09/24/internet-explorer-11-changelist-change-log.aspx