html 使用rem开发

 window.onload = function () {
            var ohtml = document.getElementsByTagName("html")[0];
            var widthMax = document.documentElement.clientWidth;
            if (widthMax > 750) {
                ohtml.style.fontSize = '100px';
            }
            else {
                ohtml.style.fontSize = (widthMax / 750) * 100 + 'px';
            }
        }

html 默认字体 100px