jsFiddle

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• JavaScript

/* webkit only */

var n = 415, 
    wrapper = $('wrapper'), 
    l = [, 371, 372, 373, 374, 375, 376, 377, 347, 324, 301, 278, 256, 378, 389, 367, 344, 321, 298, 274, 251, 227, 204, 180, 157, 133, 109, 234, 211, 188, 166, 167, 191, 169, 146, 124, 102, 80, 81, 82, 83, 108, 84, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388,, 189, 190, 213, 236, 259, 282, 305, 328, 351, 350, 327, 304, 281, 258, 235, 212, 257, 279, 302, 325, 348, 349, 326, 303, 280, 329, 352, 260, 237, 214, 192, 215, 238, 284, 307, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 343, 342, 341, 340, 336, 335, 334, 312, 311, 310, 309, 308, 285, 286, 287, 288, 314, 315, 316, 318, 319, 320, 297, 296, 273, 250, 249, 271, 270, 269, 268, 267, 266, 264, 263, 262, 290, 291, 292, 293, 294, 244, 243, 242, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 203, 202, 201, 200, 199, 198, 197, 196, 195, 194, 193, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 156, 155, 154, 153, 152, 151, 150, 149, 148, 147, 125, 126, 127, 128, 129, 130, 131, 132, 107, 106, 105, 104, 103];

for (var i = 1; i < n; ++i){
    new Element('span', {
        'class': (l.contains(i) ? '' : 'boo')
    }).inject(wrapper);
}

var b = $$('span');
(function(){
    var c = this[1][$random(0, this[0])];
    if (c && c.hasClass('boo')){
        c.addClass('hover');
    } 
}).periodical(4, [n, b]);