radial-gradient uses a comma instead of a slash to disambiguate between position and size, thus if you specify an explicit size, you must specify a position as well to disambiguate the positioning ...
We've agreed that radial gradients should have a literate syntax. Linear gradients already kinda have one, in a simple way, and hopefully radials should end up thematically consistent.
Innovation Center for Textile Science and Technology, College of Textiles, Donghua University, Shanghai 201620, China ...