i use html and css, with display flex

Your design is dedicated only for your working screen resolution. On my screen elements are out of the main white box and they are in inappropriate position to eachother. This is effect of not placing all childs of main container in the same level of DOM tree. Whereby flex positioning is not working anymore. You need to adapt your design for mobile devices as well using @media property.

Wish I helped if you want some links for tutorials on materials text me. :>