None of these solutions worked for me inside a Weebly "add your own html" box. Not sure what they are doing with their code. But I found this solution at https://benmarshall.me/responsive-iframes/ and it works perfectly.
CSS
.iframe-container {
overflow: hidden;
padding-top: 56.25%;
position: relative;
}
.iframe-container iframe {
border: 0;
height: 100%;
left: 0;
position: absolute;
top: 0;
width: 100%;
}
/* 4x3 Aspect Ratio */
.iframe-container-4x3 {
padding-top: 75%;
}
HTML
<div class="iframe-container">
<iframe src="https://player.vimeo.com/video/106466360" allowfullscreen></iframe>
</div>