• Top: SVG gradient with constant stop-color and transitioned stop-opacity;
• 2nd: SVG gradient with stop-color transitioning to black and stop-opacity transitioning to zero;
• 3rd: SVG gradient with rgba colors;
• Bottom: CSS gradient with the same rgba colors.

All transition from opaque lime to fully transparent; in all but the first SVG gradient, the final stop is transparent black. The CSS gradient scales the intensity of the color by the alpha value before transitioning, so you don't get the fade to gray effect.