On first glance it would seem that the first div has too little to go on to determine it’s rendered size, ie you set a height in CSS but no width or structural declaration. Have you tried applying width: 500px;display: block to it?
This isn’t a problem with Grav. If you google “empty div not displaying” and variants, you will see this everywhere. Have you tested on a couple different browsers? Can you not insert a nbsp; if the field is empty? Have you tried min-height? It worked fine for me (but no image, of course, in Chrome).
min-height does nothing in my case, so i will insert nbsp; for now. I will make some tries after reading some posts, i was unaware of that case of empty div, every day you learn something. thanks again for putting me on the right track