For linear gradient
A color-stop's value, followed by one or two optional stop positions, (each being either a or a along the gradient's axis).
For radial gradients
A color-stop's value, followed by an one or two optional stop positions (either a or a along the gradient's axis). A percentage of 0%, or a length of 0, represents the center of the gradient; the value 100% represents the intersection of the ending shape with the virtual gradient ray. Percentage values in between are linearly positioned on the gradient ray. Including two stop positions is equivalent to declaring two color stops with the same color at the two positions.
For conic gradients
A color-stop's value, followed by one or two optional stop positions.