I'm very rusty with CSS and having a lot of trouble with this. I have a header image and a video (I'll focus on the header image, I'm guessing the solution will also work for the video).

You could do that using an image editor, but CSS can often get the job done faster. Learn how to add a CSS reference to an image and you can stretch it horizontally to any width you like.

which is decoded directly in CSS (no need for any JavaScript) to dynamically generate an image that renders immediately. Another benefit is that without any need for wrappers or long strings of ...