The caption is not visible

[bekanntmacher]

[zoglo]

Maybe something like this:

.content-youtube figure.aspect figcaption,
.content-vimeo figure.aspect figcaption {
    position: absolute;
    /* Because the iframe itself has a border of 2px */
    bottom: 2px; 
    left: 2px;
    right: 2px;
    
    background: linear-gradient(to top, rgba(0, 0, 0, .7), transparent);
    color: #fff:

    padding: 2px;

    text-align: center;

    /* Same z-index as the iframe */
    z-index: 5;
}