code: public String attr(String attributeKey) {
          Validate.notNull(attributeKey);
          if (!hasAttributes())
              return EmptyString;
  
          String val = attributes().getIgnoreCase(attributeKey);
          if (val.length() > 0)
              return val;
          else if (attributeKey.startsWith("abs:"))
              return absUrl(attributeKey.substring("abs:".length()));
          else return "";
      }
docs: Get an attribute's value by its key. <b>Case insensitive</b>
  <p>
  To get an absolute URL from an attribute that may be a relative URL, prefix the key with <code><b>abs</b></code>,
  which is a shortcut to the {@link #absUrl} method.
  </p>
  E.g.:
  <blockquote><code>String url = a.attr("abs:href");</code></blockquote>
  
  @param attributeKey The attribute key.
  @return The attribute, or empty string if not present (to avoid nulls).
  @see #attributes()
  @see #hasAttr(String)
  @see #absUrl(String)