CSS is always generated for all browsers, but the generated gradient may look different in older browsers (depends on the features - using two or more stop points, explicit or implicit sizes).

Simple mode

IE6+, Android 2.3+, iOS 3.2+

Only two point linear gradients are supported with horizontal or vertical direction.

Utilizes only IE filter, SVG and old webkit safe features.

Advanced mode

IE9+, Android 3.0+, iOS 3.2+, WP7.5+

Supports multiple stop points, diagonal linear gradients and implicitly sized radial ellipse gradients.

Utilizes only SVG safe features.

Expert mode

IE10+, Android 4.0+, iOS 5.0+, Chrome 26+, Firefox 16+, Opera 12.1+

These browsers are supporting all CSS3 standard features (explicit radial gradient sizes, explicit center position, angled linear gradients).

Utilizes all CSS3 features.