Resize the browser window to see how the text size scales.
Use the "vw" unit when sizing the text. 10vw will set the size to 10% of the viewport width.
Viewport is the browser window size. 1vw = 1% of viewport width. If the viewport is 50cm wide, 1vw is 0.5cm.
BUT This is text when it is set to be 21pt text all the time no matter what the size of the browser window. Uggghhhh! Way too big for the normal paragraph on a desktop!! And on a phone not practical at all!!