Except that flags do not all have the same ratio. It may be useful to assume so for graphic layouts, but it is not really correct.
Unfortunately they chose 3:4 for the rectangular variants, which isn't really used that much, 2:3 and 1:2 are more common for flags[1], so lots more flags have their designs rearranged, squished and stretched to fit the box than would otherwise need to be. For example flags with the UK's Union Jack in the canton seem to be noticeably mangled as the Union Jack seems far too large relatively. It's always going to be a compromise to have one shape, but just using 2:3 (the most common shape) would mean far fewer would need adjustments.
[1] https://en.wikipedia.org/wiki/List_of_aspect_ratios_of_natio...
So the UI people just make up fake flags then. Cool.
I don't know whether SVG can handle that, but you can get JavaScript to display it correctly in any aspect ratio, of course.
It depends on calc(), vw and vh units, so it’ll work in browsers but not elsewhere.
(Certainly not all flags with similar specifications will be achievable; this one’s diagonals just happened to be all neatly anchored in the corners, though the red diagonal St Patrick’s cross is fiddly, requiring masking to achieve the required stroke alignment, for which see also https://svgwg.org/specs/strokes/#SpecifyingStrokeAlignment, which is an incomplete draft, and unimplemented.)