// styleFloat, IE
// cssFloat , Safari ou Firefox
// styleFloat and cssFloat,  Opera

//assignation  
var el = document.getElementById('element_id');
el.style.styleFloat = 'left';
el.style.cssFloat = 'left';
//récupération
var elFloat = (typeof el.style.cssFloat === 'string') ? el.style.cssFloat : el.style.styleFloat;