var div = document.createElement('div');
div.style.cssText = 'display:none;position:fixed;z-index:100;';
body.appendChild(div);
console.log(window.getComputedStyle(div).position != 'fixed');

function isSupportFixed() {
    var userAgent = window.navigator.userAgent, 
        ios = userAgent.match(/(iPad|iPhone|iPod)\s+OS\s([\d_\.]+)/),
        ios5below = ios && ios[2] && (parseInt(ios[2].replace(/_/g, '.'), 10) < 5),
        operaMini = /Opera Mini/i.test(userAgent),
        body = document.body,
        div, isFixed;

    div = document.createElement('div');
    div.style.cssText = 'display:none;position:fixed;z-index:100;';
    body.appendChild(div);
    isFixed = window.getComputedStyle(div).position != 'fixed';
    body.removeChild(div);
    div = null;

    return !!(isFixed || ios5below || operaMini);
}