Here is how to set your fonts by the screen size / viewport size with CSS

1vw = 1% of viewport width

1vh = 1% of viewport height

1vmin = 1vw or 1vh, whichever is smaller

1vmax = 1vw or 1vh, whichever is larger

These simple lines of CSS can give your site truly responsive text with just CSS:-

For Example

6vw
6vh
6vmin
6vmax

‍Note that for exactness you can also use decimals like 'font-size: 5.7vw;'.Resize this page to see the above chnage size according to the area visible on screen.

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