跨域
a网站主页

<script>
    window.onload = function () {
        var clientWidth = document.body.clientWidth + 'px';
        window.frames[0].postMessage(clientWidth, 'https://www.b.com/');
    };
</script>

b网站子页

<script>
    window.addEventListener('message', function (e) {
        if (e.source != window.parent) return;
        var clientWidth = e.data;
        $('body').width(clientWidth);
        localStorage.setItem('clientWidth', clientWidth);
    }, false);

    var clientWidth = localStorage.getItem("clientWidth");
    if (clientWidth) {
        $('body').width(clientWidth);
    }
</script>