Not the creator, but looking at the source - everything is sized using 'rem' units. (Height, width, positioning of all the elements.) Kind of like using a percentage.

    style="height: 0.160992rem; width: 1.00383rem; left: 0.5rem; top: 0.273827rem; ..."

This can be used because `font-size` on the root html tag is set to the width in pixels of the screen - or 600px maximum.

And then font sizes for all the elements are specified individually.