During my migration from V13 to V14, I encountered the problem of defining images as background-image within a css file.
Assume I had a frontend folder that has two sub folders img and styles.
Until now, the css files (actually html files) were in the described frontend folder together with the images. An easy usage of a relative url was possible to make it work: background-image: url(../img/my-bg.jpg);
But now, the images are in a completely different folder, because the css files were moved near the project root while the images stay where they were ([Here is an overview]
(https://stackoverflow.com/a/57553974/3441504)). Therefore the background image is not displayed anymore.
By trial and error I found out how to write that image path so it works in V14: background-image: url(frontend/img/my-bg.jpg);
Because I did not see any mention of this, I just wanted to share my findings and hope that it helps others.