Sometimes it’s useful to switch the browser’s default tabbing behavior (left to right) to the opposite (top to bottom) when your input fields are in a grid layout instead the of the usual single column layout. Having to do this manually is a real pain, especially for large grids; So here is a solution in javascript, using mootools:

window.addEvent('domready', function(){
    var trs = $$('#mytable tr')
    var accum = 0
    trs.each(function(tr, trindex){
        accum = trindex + 1
        tr.getChildren().each(function(td, tdindex){
            td.getChildren('input')[0].tabIndex = accum
            accum = accum + trs.length