Does webflow not support it or do you have to make the video smaller? If you place a video background on destop and tablet it works normal. Switching to lanscape or moible its just blank, is this normal? Do we have to redo the whole mobile version?
Background videos are not available on mobile. Instead you should use an animated gif. One of the reasons is that background videos eat a lot of data and its unavailability is to prevent big data loss for something the user have not actively decided to get on with (like auto played video backgrounds).
A little note though, I worked with a video on a few megabites that turned to 10 megabites in gif format, so have a close eye on that . It´s good to have those mobile users in mind.