Instead of using em or px to size text, you can use viewport units for responsive typography. Viewport units, like percentages, are relative units, but they are based off different items. Viewport units are relative to the viewport dimensions (width or height) of a device, and percentages are relative to the size of the parent container element.
The four different viewport units are:
vw(viewport width):10vwwould be 10% of the viewport's width.vh(viewport height):3vhwould be 3% of the viewport's height.vmin(viewport minimum):70vminwould be 70% of the viewport's smaller dimension (height or width).vmax(viewport maximum):100vmaxwould be 100% of the viewport's bigger dimension (height or width).
Here is an example that sets a body tag to 30% of the viewport's width.
body { width: 30vw; }