Round Corner Video

SOURCE, DEMO

The trick is to hide the poster image and use the background as cover instead because the poster can't be rounded:

CSS:

video {
   width:305px; //width or height can't be left out to keep aspect ratio
   height:160px; 
   border-radius: 8px;
   background:transparent url('your_poster_image.jpg') no-repeat 0 0;
   -webkit-background-size:cover;
   -moz-background-size:cover;
   -o-background-size:cover;
   background-size:cover;
}

HTML:

<video controls poster="transparent.png">
   <source src="parrots.mp4" type="video/mp4">
   <source src="parrots.webm" type="video/webm">
</video>

The transparent image can be download from here.