I have the same problem, youtube and vimeo video.

Thanks

Looking with ispector of chrome i've see that the border should be located in this part of code:
But i cannot fix it by myself

iframe:not([seamless]){

border: 2px inset;

border-top-color: initial;
border-top-style: inset;
border-top-width: 2px;
border-right-color: initial;
border-right-style: inset;
border-right-width: 2px;
border-bottom-color: initial;
border-bottom-style: inset;
border-bottom-width: 2px;
border-left-color: initial;
border-left-style: inset;
border-left-width: 2px;

border-image-source: initial;
border-image-slice: initial;
border-image-width: initial;
border-image-outset: initial;
border-image-repeat: initial;

}