The e function runs PHP's htmlspecialchars function with the double_encode option set to true by default:
<?php echo e('<html>foo</html>');
<?php     public function gravatar($email, $size = null)

    {

        $url = 'https://www.gravatar.com/avatar/'.e(md5(strtolower($email)));



        if ($size) {

            $url .= '?s='.$size;
<?php         }



        if (! is_null($value)) {

            return $key.'="'.e($value, false).'"';

        }

    }
<?php         if (is_array($value)) {

            return static::nestedListing($key, $type, $value);

        } else {

            return '<li>'.e($value, false).'</li>';

        }

    }