Bootstrap (https://getbootstrap.com/) is a very popular toolkit for building websites and applications. One of the key factors of its success has always been the ability to produce responsive web pages (usable both from desktop and mobile devices) with a single code base.
The most common example when talking about visual responsiveness is the Bootstrap's grid system (https://getbootstrap.com/docs/4.5/layout/grid/). It is based on a kind of abstraction of size metrics that relate every sizing to a twelve-column system. Element sizes are expressed using a number between 1 and 12 and this number is used to size the element in relative terms with respect to its container (so, 6 means half of the container). Many elements can be used as containers and nested, but the root container is (generally) the browser window (on mobile devices, basically the same size as the screen). So far, it simply sounds like a relative sizing system, but another dimension...