Bootstrap and bootstrap_package 

Decide which system supplies the site's Bootstrap CSS before adding StyleX. This changes CSS imports and template ownership, not the base connector or manifest setup in Installation.

Site Bootstrap CSS source StyleX work
New site using plain Bootstrap Import bootstrap/dist/css/bootstrap.min.css in the Vite entrypoint. Style the site's own templates and Content Blocks.
Existing bootstrap_package site with selected overrides Keep bootstrap_package CSS and templates. Override only selected templates or add new components.
Site replacing many bootstrap_package templates Keep its CSS while the replacement is incomplete. Move components to StyleX gradually and remove unused CSS deliberately.

For plain Bootstrap, install the npm package and import its CSS once in Main.entry.js. For bootstrap_package, keep its sitepackage and Site Set dependencies, and import only the JavaScript or additional styles that the project needs. Check whether the existing theme already loads Bootstrap JavaScript before importing it through Vite too. The asset collector call in Connect the build to TYPO3 remains the entrypoint for StyleX in either case.

Bootstrap and cascade layers 

Put the intended layer order at the beginning of the consuming CSS build:

@layer bootstrap, stylex, overrides;
Copied!

This declaration does not move Bootstrap CSS into a layer. For normal CSS rules, unlayered CSS wins over layered CSS regardless of selector specificity. A typical Bootstrap import and bootstrap_package output are unlayered, so use useCSSLayers: false in the StyleX Vite plugin for those setups. The base Vite example does this.

Enable StyleX layers only when the project also compiles Bootstrap into @layer bootstrap. Check the emitted CSS and computed styles before relying on that order. If TYPO3 includes CSS through the connector's optional TypoScript path, the overrideBootstrapPriority compatibility setting uses forceOnTop for the known asset names bootstrap, bootstrap_package_bootstrap, and bootstrap_cdn. See CSS delivery settings for all related settings.