The grammar for the border-image shorthand says two border-image-width values should be parsed. It seems like only one border-image-width value should be needed here.

The current grammar is: <'border-image-source'> || <'border-image-slice'> [ / <'border-image-width'> | / <'border-image-width'>? / <'border-image-outset'> ]? || <'border-image-repeat'>

It seems like what's needed is: <'border-image-source'> || <'border-image-slice'> [ / <'border-image-width'>? / <'border-image-outset'> ]? || <'border-image-repeat'>.