x1, y1, x2 and y2 are the coordinates of 2 points defining a gradient vector for the linear gradient.

These coordinates can be expressed in pixel relating to the origine, or in percent (like in this example).

The linear gradient can contain 2 or more stop elements defining the colors.

Most often, radial gradients are simple ones, that is cx=fx & cy=fy.

In the first exemple cx=fx & cy=fy, and in the second one, there is:

cx=130 cy=90 fx=32 fy=45.