Your content must be visible and accessible on any device size
Identify when to change your structure and how it will look
The size of your text on each device size
Change in selection area
The area for a user to select an element e.g. text field or button
Minimising content and adding to it at the right screen size
Vector files for images
An SVG allows any visual to be at 100% resolution at any size. One file fits all device sizes in being perfect in quality.