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", "fit-content"
or "inherit", a number, or a length calculated by the "calc"
CSS function. If the number has a suffix, it must be valid: px,
\%, ch, em, rem, pt, in, cm,
mm, ex, pc, vh, vw, vmin, or
vmax.
If the number has no suffix, the suffix "px" is appended.
Any other value will cause an error to be thrown.