Avoid unnecessary legacy transforms and polyfills in modern reader bundles #130

Problem

Modern browsers receive compatibility code Lighthouse identifies as unnecessary for the measured browser.

Production audit evidence

Measured on 2026-10-07 UTC against https://devfeed.tech using Lighthouse 12.6.1 and Chrome 154.0.8037.97, with fresh anonymous browser profiles. Mobile uses the default simulated mobile network/4× CPU profile (412×823, DPR 1.75); desktop uses the default desktop profile (1350×940, DPR 1). These are lab observations, not field p75 data.

legacy-javascript repeatedly estimated 24,356 bytes (24KiB) waste on homepage runs; the newer legacy insight identified roughly 14KiB in one first-party chunk. Signals include Array.at/flat/flatMap, Object.fromEntries/hasOwn, String.trimStart/trimEnd, and transpiled classes. The two audit estimates are alternate analyses and must not be summed.

Audit IDs: legacy-javascript, legacy-javascript-insight.

Work required

• Trace flagged output to build targets, framework polyfill behavior, and dependencies. Define the actual supported-browser baseline before adjusting transforms.
• Use supported build options and dependency choices; do not patch generated bundles or remove polyfills needed by supported browsers.

Acceptance criteria

• Modern production bundles have fewer measured legacy/polyfill bytes.
• Tests confirm the defined supported browser matrix, including Chrome and Edge extensions, and cover code paths depending on flagged APIs.
• Capture comparable before/after reports and add regression coverage for the changed behavior. For shared reader changes, verify browser behavior on web and both built Chrome/Edge extensions.