Hi @MathieuTrygr, Dozie from Memberstack here
Saw your question hadn’t been fully responded to and wanted to share info I think might help 
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1 - To use the position parameters, try making sure your video is absolutely positioned within a div that is relatively positioned.
Div
Video
When you get it right you’ll see the absolutely positioned video element will say what it’s positioned relative to.
2 - But that may not fix your problem of a video overflow. I suggest having a look at this conversation where they had a similar issue
To properly set a custom width and height for a video element, you need to place it in a div, and set the width and height of that parent div in pixels. I suggest avoid using percentages.
Step 1
Add Div
Step 2
Set custom width and height
Step 3
Place a video element. The video element will take up the full width of the custom div, but you might need to change the size of the custom div for each device size.
You can see full details in this Webflow article