等比缩放H5页面

! function( userAgent ) {

    var screen_w = parseInt(window.screen.width),

        scale = screen_w / 640;

    if ( /Android (\d+\.\d+)/.test( userAgent ) ) {

        var version = parseFloat( RegExp.$1 );

        document.write( version > 2.3

            ? '<meta name="viewport" content="width=640, minimum-scale = ' + scale + ", maximum-scale = " + scale + ', target-densitydpi=device-dpi">'

            : '<meta name="viewport" content="width=640, target-densitydpi=device-dpi">' );

    } else {

        document.write( '<meta name="viewport" content="width=640, user-scalable=no, target-densitydpi=device-dpi">' );

    }

}( navigator.userAgent );