The CanvasGradient interface represents an opaque object describing a gradient. It is returned by the methods CanvasRenderingContext2D.createLinearGradient() or CanvasRenderingContext2D.createRadialGradient().
The CanvasGradient.addColorStop() method adds a new stop, defined by an offset and a color, to the gradient. If the offset is not between 0 and 1, an INDEX_SIZE_ERR is raised, if the color can't be parsed as a CSS <color>, a SYNTAX_ERR is raised.