It might work with very simple apps that have lot of excess whitespace on larger devices. You'd focus on the mobile experience first, and then on blowing it up

It might work with very simple apps that have lot of excess whitespace on larger devices. You'd focus on the mobile experience first, and then on blowing it up in a visual pleasing way. For the layout you'd use a rule based approach instead of breakpoints. But yeah, vanilla css folks are more focused on content sites like blogs, not web-apps with dense tables, dashboards etc.