code: public Node attr(String attributeKey, String attributeValue) {
          attributeKey = NodeUtils.parser(this).settings().normalizeAttribute(attributeKey);
          attributes().putIgnoreCase(attributeKey, attributeValue);
          return this;
      }
docs: Set an attribute (key=value). If the attribute already exists, it is replaced. The attribute key comparison is
  <b>case insensitive</b>. The key will be set with case sensitivity as set in the parser settings.
  @param attributeKey The attribute key.
  @param attributeValue The attribute value.
  @return this (for chaining)