You support a fallback which redirects for unsupported browsers.
You support a fallback which loads and alternative experience inline when lack of support is detected.
The Firefox browser produced by Mozilla
The Chrome browser produced by Google
The Safari browser produced by Apple
The Edge browser produced by Microsoft
The Node.js runtime produced by theOpenJS Foundation.
You write everything inside one or more modules which are directly included or inline in the HTML, and publish that directly.
You use a variety of dependencies, which are all native es modules and include them using an importmap.
You use browser compatible modules in your development process and can run locally without build, but for delivery you compile them to an output format.
The target format you export for the browser.