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I am talking about Responsive Web Design
We can achieve responsive design in different way. You can create it in complex way or simple it is upto your css skills.
1) If you play wisely in css properties and handle with percentages then one same css can be used for all devices. This needs less testing. All devices will handle automatically.
2) Other practice is to handle with different media sizes. Means complex css code development, more testing to check on countless devices.
If you're organized, you'll find things easy at all times. Know the range of devices you're planning to design for, and jot them down. Once you prepare this information you can use it multiple times for different designs. After a few designs you'll have developed a routine and that will make things easier.
This depends on the web project you are working on.