js根據屏幕寬度計算rem比例

function fnInitScreen() {
    var html = document.getElementsByTagName("html")[0];
    var width = window.innerWidth;
    var font_size = 0;
    font_size = 100/1500*width;   //100 設計稿單位計算的分母(400/100)=4rem    1500 設計圖比例
    html.style.fontSize = font_size + "px";
}
fnInitScreen();
window.onresize = function () {
    fnInitScreen();
}