<style>
    .responsive-iframe {
        position: relative;
        width: 100%;
        padding-top: 56.25%; /* 16:9 or 4:3 aspect ratio */
    }

    .responsive-iframe iframe {
        position: absolute;
        top: 0;
        left: 0;
        width: 100%;
        height: 100%;
    }
</style>

<div class="responsive-iframe">
    <iframe src="https://www.domain.com/?form=333" style="border: none;" scrolling="no"></iframe>
</div>