Place the add-on's frontend bundle sources in this directory. They are
packaged into the JAR and used as input when the consuming application
builds its frontend bundle. This is the place for JS modules referenced
with @JsModule, and for CSS files that should be included in the bundle
and loaded with @CssImport.

Use the ./ prefix when referencing the files, e.g.
@JsModule("./my-addon/my-module.js") for
src/main/resources/META-INF/frontend/my-addon/my-module.js

CSS is typically not bundled: place stylesheets in
src/main/resources/META-INF/resources and load them with @StyleSheet.
Static resources, such as images, also go there.
