The specification for URLs (RFC 1738, Dec. '94) poses a problem, in that it limits the use of allowed characters in URLs to only a limited subset of the US-ASCII character set:

"...Only alphanumerics [0-9a-zA-Z], the special characters "$-_.+!*'()," [not including the quotes - ed], and reserved characters used for their reserved purposes may be used unencoded within a URL."

HTML, on the other hand, allows the entire range of the ISO-8859-1 (ISO-Latin) character set to be used in documents - and HTML4 expands the allowable range to include all of the Unicode character set as well. In the case of non-ISO-8859-1 characters (characters above FF hex/255 decimal in the Unicode set), they just can not be used in URLs, because there is no safe way to specify character set information in the URL content yet [RFC2396.]