The 'mobile-first' approach in UX/UI design involves designing websites and applications first with mobile devices in mind, which is crucial due to the growing popularity of browsing the Internet on smartphones and tablets. This design method ensures that user experiences are optimal on smaller screens and then adapted to larger devices.

The key principles of good UX/UI design include simplicity, readability, intuitiveness, consistency and accessibility. It is important that the design is easy to use, with easily accessible information, a visually appealing interface, and that it provides a positive experience for users with different needs and skills.

The effectiveness of UX/UI design can be measured by analyzing indicators such as conversion rate, time spent on the website, bounce rate, and through direct user opinions. Analytical tools and usability testing help you understand how users interact with your design and where improvements can be made.

Collaboration between designers and developers is crucial to the success of a UX/UI project. It ensures smooth communication, understanding and implementation of the design vision, and effective implementation of functions. Proper coordination between these two groups is essential to creating coherent, functional and aesthetic digital products.