via make.wordpress.org => original post link
Gutenberg 13.8 introduced the first version of a fluid typography feature, which allows theme authors to define font sizes that adapt to the viewport. Version 13.9 will also add a couple of fixes and enhancements.
Because this is a new feature that will primarily affect how theme authors build their designs, it needs thorough testing from the theme development community. Please take some time to test and share feedback in the comments.
Overview of Fluid Typography