Webflow Expert Designer uses REM (root em) as one of the unit types for sizing elements. REM is relative to the font-size of the root element of the document, which is typically set to 16px.
Using REM units enables designers to create responsive websites much easier and faster. When using REM, all elements on the page are sized relative to the root font-size, making it easy to make global changes to the size of elements on the page.
In addition, REM units help maintain accessibility, as they allow users to change the font size of their browser without affecting the layout of the website.
While Webflow Designer does allow the use of px units, it is recommended to use REM units for a more flexible and responsive design.