<video autoplay poster="test.jpg" id="vidos">
<source src="test.mp4" type="video/mp4">
</video>
#vidos {
position: fixed;
right: 0;
bottom: 0;
min-width: 100%;
min-height: 100%;
width: auto;
height: auto;
z-index: -1;
background: url(test.jpg) no-repeat;
background-size: cover;
}