Bootstrap 6 Alpha is a major modernization of the framework that ships as an npm/CDN alpha and requires modern browsers (Chrome/Edge 130+, Firefox 132+, Safari 18+). It moves to ESM-only JavaScript (script tags must use type="module"), raises the support floor so there are no fallbacks or polyfills for newer CSS features, and keeps Bootstrap 5 available for older targets. The release highlights deep rewrites, updated build tooling, contributor acknowledgments, and migration guidance for projects moving from v5.
The most important technical changes are a Sass-module-first workflow and a CSS-first customization model: nearly every former Sass variable is now a CSS custom property organized into token maps (global $root-tokens and per-component maps) that can be merged and overridden at build time or runtime. @use/@forward replace @import, and PostCSS now injects a configurable prefix (e.g., md:col-6) for responsive and state utilities instead of v5’s infix. The codebase embraces modern CSS primitives - CSS Layers, OKLCH and color-mix, light-dark(), :has(), container and range queries, @property, content-visibility with transitions, and native details/dialog - enabling pure-CSS components and more predictable theming while reducing JavaScript surface area.
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