The desktop_light segment basically covers all older desktop browsers as far back as the earliest Netscapes and Internet Explorers. It's every desktop browser that doesn't pass the desktop criteria. It is considered a fallback segment to ensure even old browsers can be supported if so desired.
As of December 2017 this segment represents ~6% of the desktop browsers and ~3% of all browsers (across all device types).
The desktop_light segment is defined by the following criteria:
- Primary input method is mouse or trackpad
- Browser supports the desktop_light segment requirements
- Screen width of a minimum of 800 pixels
The desktop_light segment supports the following features:
- HTML 4
- Some CSS
To get a hint at the feature support check out caniuse.com.