NULL and NA are returned unchanged.
Single element numeric vectors are returned as a character vector with the
number plus a suffix of "px".
Single element character vectors must be "auto" or "inherit",
or a number. If the number has a suffix, it must be valid: px,
%, em, pt, in, cm, mm, ex,
or pc. If the number has no suffix, the suffix "px" is
appended.
Any other value will cause an error to be thrown.