A mobile viewer might have to scroll horizontally to read this second box, which isn't standard, but it's doable. A modern flexbox layout, while it is powerful enough to transform the page for mobile viewers, if the designer fails to think about this, it will transform the layout such that mobile viewers can't consume it. With the "sidebar" on the left, we can think of it as the primary content space. Anything to the right is secondary, for example this box is a continuation of the content to the left.