Configuring Browser.element for zero‑downtime incremental Elm migration alongside legacy JavaScript

Goal: embed an Elm application into an existing server‑rendered page using Browser.element, serve the compiled elm.js as a static asset, and load it conditionally via a feature flag so the legacy JavaScript continues to run unchanged, enabling a zero‑downtime migration of a small frontend feature.

Constraints/uncertainty: Browser.element offers no CSS or DOM encapsulation, meaning global stylesheets or third‑party scripts can leak into or alter the Elm‑rendered subtree; port communication lacks compile‑time enforcement on the JavaScript side, which can result in exceptions or silent message drops if the Elm and JS handlers diverge; achieving true zero‑downtime relies on deployment mechanics such as conditional script loading or feature flags, which Elm does not provide natively.

Questions: What approaches can isolate Elm‑generated CSS from global stylesheets when using Browser.element? How can teams enforce reliable, type‑safe port contracts across the Elm‑JS boundary without runtime errors? Is there a recommended pattern for coordinating feature‑flagged script tags with Browser.element mounts to avoid duplicate initializations or state loss?