You mean stretch the aspect ratio of a video element to whatever size the container is? No, the video will resize to fit in the browser while still retaining its aspect ratio - this is actually desirable in most cases.
It is possible to do what you want using the object-fit css property, but it's currently only supported in the latest versions of Chrome and Opera browsers:
Is there a more crossbrowser way to do this? seems the support of that property is less than 40%