Custom properties, initially introduced as "variables" in the CSS Variables Module in 2012, have evolved into a versatile tool for defining responsive behavior in web design, despite the module still being a candidate recommendation since 2015. These properties, which follow the CSS cascade, can be used in any CSS block and are referenced through the var() function. While their direct use might not be evident when building custom behaviors, they shine when integrated into frameworks, acting as an API that allows users to control behavior without delving into the underlying code. LayerJS serves as a prime example, where custom properties assist in creating responsive, animated user interfaces by defining dynamic, overlapping layers controlled by HTML attributes or CSS properties. This approach empowers users to tailor layouts and behaviors across different devices, enhancing responsiveness without imposing predefined assumptions. The utility of custom properties extends beyond aesthetics, streamlining the creation of adaptable UIs within frameworks like layerJS, which accommodates user-defined responsiveness through a grid system or breakpoints, thereby offering a flexible and efficient means of defining interface behavior.