fr:
The <flex> CSS data type denotes a flexible length within a grid container. It is used in grid-template-columns, grid-template-rows and other related properties.
p:
One pixel. For screen displays, it traditionally represents one device pixel (dot). However, for printersand high-resolution screens, one CSS pixel implies multiple device pixels. 1px = 1/96th of 1in.
em:
Represents the calculated font-size of the element. If used on the font-size property itself, it represents the inherited font-size of the element.
rem:
Represents the font-size of the root element (typically <html>). When used within the root element font-size, it represents its initial value (a common browser default is 16px, but user-defined preferences may modify this).