CSS can't be easier than the concepts it describes.

If you want a completely different menu on mobile, well, you're going to have at least two menues.

Should images keep their aspect ratio or be stretched? Minimum size in pixels or as percentage? Should the image container constraint the image or should the image push the container's boundary?

I often ask people what they want to happen. Rarely can they describe it in words – how do they expect to write CSS? Of course they're going the explorative route.