(function draw(i){ return function () { document.write(i%30==0 ? '<br>' : Math.random()>0.5?'\u2571':'\u2572'); setTimeout(draw(i+1), 100) } }(1)())

A throttled run-forever version.

edit, slightly neater:

(function draw(i){ setTimeout(function (){ document.write(i%30==0 ? '<br>' : Math.random()>0.5 ? '\u2571':'\u2572'); draw(i+1); }, 100) }(1))