Validates any border-style value that contains 0-4 (valid) border styles.
Only restriction on whitespace is at least one space/tab between values—all of the examples below are valid:
"solid"
"solid none"
"none none solid"
"outset inset inset solid"
"solid none none"
"solid"
"solid"
"none solid"
"inset outset none"
"inset inset outset none"