Source code must not use require or module.exports but valid ES imports and exports
Use jsx instead of js extension for files that contain jsx
Remove ejs templates from index.html
Add your entrypoint in index.html with a script tag (for example <script type='module' src='/src/index.jsx'> if src/index.jsx is your entry)
Instead of using the package.json homepage option you should pass basePath to bundless.config.js in the build.basePath options
If using a workspace all your linked packages must expose valid ESM code (no require calls) (if this is not the case, add prebundleWorkspacePackages option)
Replace eventual webpack plugins with esbuild alternatives (notice that the most popular webpack plugins are built in in bundless so you probably don't need any additional plugins, see features to see if there is something missing)
Replace %PUBLIC% occurrences in your index.html file with just /, bundless will remap those paths to use config.build.basePath automatically