UI Toolkit is made for maximum performance and reusability with workflows and authoring tools informed by standard web technologies. UI designers and artists will likely find it familiar, especially with prior experience designing web pages.

Three major sections of the guide highlight instructions for developing runtime UI with UI Toolkit. There’s a thorough explanation of the parts that comprise UIs made with UI Toolkit, including the Unity Extensible Markup Language (UXML) and Unity Style Sheet (USS) using UI Builder.

You’ll explore how UI Toolkit positions visual elements based on Yoga, an HTML/CSS Layout engine that implements a subset of Flexbox. Flexbox architecture provides advantages, such as responsive UI, enabling you to adapt your UI to different screen resolutions and sizes. Through both UXML and USS, you can decouple the styles applied to UI layouts (and switch those styles up as needed), while logic and functionality continue to live in code. Workflows for visual elements, the fundamental building blocks of each interface, are also discussed in great detail – from positioning, size, and alignment settings, to margins and padding.

Source: Unity Technologies Blog