My use case is wanting layout items to take up a certain portion of the screen across screen resolutions. For example suppose I'm building this UI:

I want to say that the <TextBox> and <Button>s are width: 80% (or that their parent <StackLayout>) is, but currently I have to pick an arbitrary numeric amount.

I can get the behavior I seek if I create elements in JS, by getting the screen width and using that to derive percentage-based widths, but that's a lot of work. I'd love to be able to type %, in CSS as well as layouts—especially <GridLayout>.