Responsive design is more than making a page narrower. Real devices introduce different screen sizes, touch interactions, browser controls and text wrapping.
Test navigation
Make sure menus remain usable and important links are easy to tap.
Test real content
Long headings, large images and unexpected text lengths often expose problems that sample content does not.
Test landscape and portrait
A layout that works in portrait may behave differently when the phone rotates.
Prepared for Furynight by Md Millad Hossan.