Per
https://www.gnu.org/software/emacs/manual/html_node/elisp/Face-Attributes.html

(:line-width (vwidth . hwidth) :color color :style style)
You can explicitly specify all aspects of the box with a plist on this
form. Any element in this plist can be omitted.

In the phrase "with a plist", the word "on" should be "of".