After looking at some of the source code in #Sharkey's implementation of #MFM, I discovered the details of the $[border decoration.
It accepts a width property for how thick the border should be (don't specify units)
There's a radius property for corner roundness (don't specify units)
You can specify a color with a 3, 4, or 6 digit hex code (don't put a hash sign first)
You can specify style where the options are ['hidden', 'dotted', 'dashed', 'solid', 'double', 'groove', 'ridge', 'inset', 'outset'] with a default of solid if you try to put in something else.
The noclip keyword doesn't have any properties. If you don't say noclip then it will add overflow:clip to the border which hides stuff inside it if it doesn't fit.
