# Safari Technology Preview 21 Release Notes Preview the latest web technology updates including: JavaScript, Fetch API, IndexedDB 2.0, CSS, Rendering, Accessibility, Web API, Web Inspector, Security, and bug fixes. ## Overview [Safari Technology Preview](https://webkit.org/blog/6017/introducing-safari-technology-preview/) Release 21 is now [available for download](https://webkit.org/downloads/) for macOS Sierra. If you already have Safari Technology Preview installed, you can update from the Mac App Store’s Updates tab. This release covers WebKit revisions [209803-](https://trac.webkit.org/log/?stop_rev=209238&rev=209803&limit=999)[210274](https://trac.webkit.org/log/?stop_rev=209803&rev=210274&limit=472). ### JavaScript - Enabled concurrent garbage collection ([r209694](https://trac.webkit.org/changeset/209694)) - Enabled `