The desktop_ie9 segment is a somewhat sad compromise. Feature wise it does not deserve its own segment and should simply be degraded to desktop_light. Despite a low public market share some large corporations and organizations still use this browser internally and thus it is still occasionally a part of the primary support requirement. Therefore degradation is not a valid approach. Giving it its own segment seemed to be the most flexible approach as developers can then individually decide how to group it with other segments.
As of December 2017 this segment represents ~2% of the desktop browsers and ~1% of all browsers (across all device types).
The desktop_ie9 segment is defined by the following criteria:
- Primary input method is mouse or trackpad
- Screen width of a minimum of 800 pixels
- Browser is Internet Explorer 9
The desktop_ie9 segment does NOT support the following features:
- FileReader API
- File API
- CSS Transitions
- CSS Animations
- Multiple file selection
- Offline storage
- HTML5 input types
Other browsers of that age supported these features and you may need special libraries to support this browser.
Check out the complete feature support on caniuse.com.