What changed
On October 8, 2026, Bootstrap published version 6.0.0-alpha.1, its first major-version preview since Bootstrap 5. The framework now requires Chrome and Edge 130+, Firefox 132+ and Safari 18+, dropping the old browser fallbacks. It replaces most Sass variable-based visual customisation with CSS custom properties and Sass @use/@forward token maps, switches JavaScript plugins to ESM-only modules, and changes responsive utility and component variant naming from infixes such as d-md-none and col-lg-6 to prefixes such as md:d-none and lg:col-6. Native browser primitives including dialog, details/summary, container queries, :has(), oklch(), light-dark() and CSS layers replace older framework machinery.
Why it matters
Bootstrap remains an important default UI layer for many server-rendered websites, PHP apps, admin tools and design systems. This is not a routine patch: CSS class names, Sass customisation entry points, JavaScript imports and supported browser versions change together. Builders who maintain long-lived Bootstrap 5 themes or components must assess whether a future v6 migration is worth the rework and which browser populations they can leave behind. The alpha is available for experiments, not a signal to upgrade production sites now.
Responsive classes and Sass customisation are migration breaks
The new prefix system changes class selectors across grid, utilities, components and states: col-lg-6 becomes lg:col-6, and d-md-none becomes md:d-none. Sass @import and old global variables are no longer the normal extension surface. Visual tokens move into CSS custom properties, while @use/@forward and token maps generate component styles; projects with extensive Sass overrides will need a rework.
JavaScript plugins become ESM-only
Bootstrap 6 removes UMD bundles and the window.bootstrap global. CDN users must load the bundle with type=module; code calling constructors should import named APIs. Teams bundling Bootstrap need to audit how their build tool handles ESM and verify their data-attribute integrations.
The modern browser floor enables simpler native UI
The alpha requires Chrome/Edge 130, Firefox 132 and Safari 18, instead of Bootstrap 5's much older support baseline. The rewrite leans on CSS layers, container queries, native dialog, details/summary and colour APIs, without polyfills for browsers below the new floor. Bootstrap 5 remains supported; maintainers say 5.4.0 will arrive before an end-of-life date is announced.
An alpha is a migration laboratory, not general availability
Bootstrap 6.0.0-alpha.1 is installable from npm and CDN, with early documentation and a migration guide. API details can still change before a stable release. The appropriate near-term action is testing real Bootstrap 5 components and Sass overrides in a separate branch, measuring breakage and browser reach, not assuming the preview is production-ready.