Apple is offering the 122nd version of Safari Technology Preview tonight. This is the experimental version of Safari to allow developers to have a preview of what is coming soon in the browser.
Developers are encouraged to read the full changes on the Apple website. The download is done in click here for macOS Catalina and in click here for macOS Big Sur. Go to System Preferences> Software Update to download the new update if you have a previous version of Safari Technology Preview already installed.