Get or set the URL of the W3C Markup Validation service to employ.
w3c_markup_validate_baseurl(new)
a character string with the URL of the the W3C
Markup Validation service to employ, or NULL
indicating to
use the W3C service as specified by the environment variable
W3C_MARKUP_VALIDATOR_BASEURL
, or if this is unset,
at https://validator.w3.org/check.
If no argument is given, the current URL is returned.
Otherwise, the URL is set to the given one or (if new
is NULL
) the default one.