This is something I pulled my hair out over for a while, but I came across a great solution that doesn't use any script, and can achieve a perfect cover simulation on video with 5 lines of CSS (9 if you count selectors and brackets). This has 0 edge-cases in which it doesn't work perfectly, short of CSS3-compatibility.
You can see an example here (archived)
The problem with Timothy's solution, is that it doesn't handle scaling correctly. If the surrounding element is smaller than the video file, it isn't scaled down. Even if you give the video tag a tiny initial size, like 16px by 9px, auto ends up forcing it to a minimum of its native file-size. With the current top-voted solution on this page, it was impossible for me to have the video file scale down resulting in a drastic zoom effect.
If the aspect ratio of your video is known, however, such as 16:9, you can do the following:
.parent-element-to-video {
overflow: hidden;
}
video {
height: 100%;
width: 177.77777778vh; /* 100 * 16 / 9 */
min-width: 100%;
min-height: 56.25vw; /* 100 * 9 / 16 */
}
If the video's parent element is set to cover the entire page (such as position: fixed; width: 100%; height: 100vh;), then the video will, too.
If you want the video centered as well, you can use the surefire centering approach:
/* merge with above css */
.parent-element-to-video {
position: relative; /* or absolute or fixed */
}
video {
position: absolute;
left: 50%; /* % of surrounding element */
top: 50%;
transform: translate(-50%, -50%); /* % of current element */
}
Of course, vw, vh, and transform are CSS3, so if you need compatibility with much older browsers, you'll need to use script.