Unfortunately, since Safari Technology Preview is an experimental version for developers, it will take some time before the native macOS version of Safari gets updated with full ProMotion support.
For some unknown reason, Apple shipped macOS Monterey without full 120Hz support in its native apps – which is odd since ProMotion is one of the main selling points of the 2021 MacBook Pro.
In addition to 120Hz support, Safari Technology Preview 135 also comes with other enhancements and new features, as you can see below:
For users who already have Safari Technology Preview installed, the update is available through the Software Update menu within the System Preferences app. If you don’t have the Safari Technology Preview installed, it is available for download on the Apple Developer website. This version of Safari can be installed on macOS Monterey and macOS Big Sur without replacing the main Safari app.
Release notes for Safari Technology Preview 135 are available with accent-color, lazy image loading, smooth scroll animations at 120Hz, requestVideoFrameCallback API, and new viewport units including: small svw/svh, large lvw/lvh, & dynamic dvw/dvh.