Checks that the argument is valid for use as a CSS unit of length.
The unit to validate. Will be treated as a number of pixels if a unit is not specified.
NA are returned unchanged.
Single element numeric vectors are returned as a character vector with the
number plus a suffix of
Single element character vectors must be
or a number. If the number has a suffix, it must be valid:
pc. If the number has no suffix, the suffix
Any other value will cause an error to be thrown.
A properly formatted CSS unit of length, if possible. Otherwise, will throw an error.