Removing black borders on a vimeo iframe embed using CSS?

For your use case, I don't think you'll be able to use just css.

Usually we add letterboxing or pillar boxing around video iframes to keep the height and width at a certain ratio for presentation. But in that case, the black borders would just be as simple as a css background.

To keep things responsive, you would set the height to something like zero (like you have) and use the padding hack to keep the aspect ratio of the video (in this case a 16:9 video; 9/16 * 100 = 56.25%). That number would be either your padding-top or padding bottom value. Since the padding is measured with percent, this scales the padding in relation to the width keeping the correct ratio no matter what width you size the video to.

In your case, this video actually has the letterboxing in the actual video which you can see from the source of the video tag within the iframe. I'm not sure why you have the padding-top:30 but that makes the black borders even bigger. You'll need to hack your situation even more though because of the built in letterboxing. I put together a jsfiddle demo here which includes a few comments but it uses JS to achieve what you're looking for.

The concept for the code is as follows:

• You want the outer container to crop off the bottom and top of the video. Assuming you wanted the video to be responsive, and be cropped, you need to always have the actual video be larger than the outer container which masks it.
• The video should be moved up in relation to how wide the video is vs the thickness of the top border
• You'll want to shrink the height of the outer container a bit to compensate for the negative top margin yet still hide the bottom portion of the video

Personally I don't like doing expensive DOM operations on resize which maybe is the reason you asked for solely css but FWIW, you have the demo.

Ideally your best option would be to get the video re-recorded without the letterboxing so all you would need is the padding hack.

Cut the 1px off all edges with CSS:

.embed-container { 
    position: relative; 
    padding-bottom: 43%;        /* Aspect ratio of the video */
    height: 0; 
    overflow: hidden; 
    max-width: 100%;
} 

.embed-container iframe, 
.embed-container object, 
.embed-container embed { 
    position: absolute; 
    top: -1px; 
    left: -1px;
    width: calc(100% + 2px); 
    height: calc(100% + 2px);
}

HTML:

<div class="js-video [vimeo, widescreen]">
  [video html goes here]
</div>

CSS:

.js-video {
  height: 0;
  padding-top: 25px;
  padding-bottom: 67.5%;
  margin-bottom: 10px;
  position: relative;
  overflow: hidden;
}

.js-video.widescreen {
  padding-bottom: 57.25%;
}

.js-video.vimeo {
  padding-top: 0;
}

.js-video embed, .js-video iframe, .js-video object, .js-video video {
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  position: absolute;
}

You will find more details here