function orientation_change() {
var viewport = document.querySelector('meta[name="viewport"]');
if (window.orientation == 0 || window.orientation == 180)
viewport.setAttribute("content", "width=device-width, maximum-scale=1.0, initial-scale=1.0, user-scalable=no");
else if (window.orientation == -90 || window.orientation == 90)
viewport.setAttribute("content", "width=device-height, maximum-scale=1.0, initial-scale=1.0, user-scalable=no");
}
window.onorientationchange = orientation_change;
window.orientation
:这个属性给出了当前设备的屏幕方向，0表示竖屏，正负90表示横屏（向左与向右）模式
onorientationchange
: 在每次屏幕方向在横竖屏间切换后，就会触发这个window事件，用法与传统的事件类似
#b{width:300px;height:200px;background:blue;}
@media screen and (orientation:portrait){
#b{background:red;}
}
@media screen and (orientation:landscape){
#b{background:green;}
}