The <url> tag always requires an @href attribute. Usually this will be some external web page, or other resource. If the <url> is empty, then the value of the @href attribute will be the link text, typically in a monospace font. You can also provide your own content, using elements much like any other piece of text that would occur in a paragraph. Of course, there is no need to anticipate any problems with output and special characters like a tilde. Use the character itself in the @href, and use the <tilde> element in the content (or wrap the content in the <c> element and use the literal character).