On UI Elements (UI Panel, UI Button, etc), allowing the width and height to be set as percentage values (eg: 0-100% of parent) will allow for layouts to be designed that work on any screen size.
Currently, it is only possible to set absolute values for the width and height of UI Elements. Layouts designed this way will not work if the dimensions entered do not fit within the screen size. For example, if I design a UI Panel to be 1000 in width, but a mobile screen size is only 400 in width, then there will be issues.
So in preparation for Core going mobile and supporting any screen size, I think it will be necessary to have the ability to set width and height to percentage values. Adding this feature now will allow developers to design UI interfaces now that will be compatible with any screen size in the future, such as mobile screens.