Apple has posted a new beta version (108) of Safari Technology Preview. It requires macOS 10.15.

The beta offers a sneak peek at upcoming web technologies in the web browser for macOS, iPadOS, and  iOS. It allows developers to experiment with these technologies in their websites and extensions. Safari Technology Preview 108 includes bug fixes and performance improvements for WebInspector, Accessibility, Web API, CSS, JavaScript, Scrolling, Rendering, Async Clipboard API, Web Animations, and Media.