Fluid Styled Content 

Keep existing FSC styles 

For an existing site, keep typo3/fluid-styled-content and typo3/fluid-styled-content-css in the active Site Set. Override only the templates or partials whose markup should use StyleX. Set the sitepackage's template fallback paths in its Site Set settings, for example:

styles.templates.layoutRootPath: EXT:my_sitepackage/Resources/Private/ContentElements/Layouts
styles.templates.partialRootPath: EXT:my_sitepackage/Resources/Private/ContentElements/Partials
styles.templates.templateRootPath: EXT:my_sitepackage/Resources/Private/ContentElements/Templates

Copy the selected FSC template into the matching sitepackage directory, then apply stylex:class keys to its elements. Existing elements continue to use FSC's templates and CSS. Check the browser's computed styles where FSC selectors and atomic classes set the same property.

Own the presentation in the sitepackage 

If the site supplies styles for its standard elements, omit typo3/fluid-styled-content-css from the active Site Set while retaining typo3/fluid-styled-content where its content rendering is used. Author the site's templates and Content Blocks, and import their StyleX files from the Vite entrypoint. See Content Blocks for glob discovery.

Rich-text output still contains elements such as paragraphs and links that have no StyleX class on each element. Include a scoped base stylesheet in the Vite entrypoint for that output:

import '../CSS/rte.css';

.ce-bodytext p { margin-block-end: 1rem; }
.ce-bodytext a { text-decoration: underline; }

With either choice, keep useCSSLayers: false while FSC or other CSS is unlayered. The cascade guidance in Bootstrap and cascade layers explains why a layer-order declaration alone does not layer existing CSS.