I think it would be nice to have slashes and backslashes (and any custom symbols or strings, maybe, by means of Unicode) as border-style properties.
Maybe, as in the following example:
border-string: ‘border text sample’;
border-string-style: ‘Times New Roman’ 1em italic bold blue;
As well as double or even triple border (through, maybe, something like secondary-border):
secondary-border-color
secondary-border-style
secondary-border-width
secondary-border-image
secondary-border-radius
secondary-border-break
…
tertiary-border-color
tertiary-border-style
…