code: public String absUrl(String attributeKey) {
          Validate.notEmpty(attributeKey);
  
          if (!hasAttr(attributeKey)) {
              return ""; // nothing to make absolute with
          } else {
              return StringUtil.resolve(baseUri(), attr(attributeKey));
          }
      }
docs: Get an absolute URL from a URL attribute that may be relative (i.e. an <code>&lt;a href&gt;</code> or
  <code>&lt;img src&gt;</code>).
  <p>
  E.g.: <code>String absUrl = linkEl.absUrl("href");</code>
  </p>
  <p>
  If the attribute value is already absolute (i.e. it starts with a protocol, like
  <code>http://</code> or <code>https://</code> etc), and it successfully parses as a URL, the attribute is
  returned directly. Otherwise, it is treated as a URL relative to the element's {@link #baseUri}, and made
  absolute using that.
  </p>
  <p>
  As an alternate, you can use the {@link #attr} method with the <code>abs:</code> prefix, e.g.:
  <code>String absUrl = linkEl.attr("abs:href");</code>
  </p>
  
  @param attributeKey The attribute key
  @return An absolute URL if one could be made, or an empty string (not null) if the attribute was missing or
  could not be made successfully into a URL.
  @see #attr
  @see java.net.URL#URL(java.net.URL, String)