Legacy Browser EOL Migration Checklist
A migration checklist for dropping end-of-life browser support. It covers analytics-based decisions, baseline policy, build-target updates, graceful degradation, and post-launch monitoring.
When to Use This Checklist
Use this checklist when dropping support for end-of-life (EOL) browsers, such as legacy versions that no longer receive updates. Removing them lets you ship modern JavaScript and CSS, shrink bundles by dropping polyfills, and reduce test surface. The risk is alienating real users, so plan the change with data.
How to Use This Checklist
Start with analytics. Quantify how much traffic and revenue still come from EOL browsers before deciding to drop them. A baseline policy should be a business decision informed by real numbers, not just an engineering preference.
Once the baseline is set, capture the technical win. Audit polyfills and transpilation targets tied to old browsers, then update build targets to the new baseline; this often produces a meaningful bundle-size and performance improvement. Replace legacy APIs and vendor-prefixed CSS that the new baseline makes unnecessary.
Protect affected users. Show a clear upgrade message to unsupported browsers, ensure critical flows degrade gracefully, and test across the new matrix. After launch, watch error rates and conversions for affected segments so you can react if the impact is larger than expected.
What Good Looks Like
The decision is backed by analytics, a supported-browser policy is documented, and build targets match the new baseline for a smaller bundle. Unsupported browsers see a clear upgrade message, critical flows degrade gracefully, and the new matrix is tested. Post-launch monitoring tracks error rates and conversions for affected segments.
Common Pitfalls
The biggest pitfall is dropping browsers without data and silently losing a meaningful user segment. Teams also remove polyfills but forget to update build targets, leaving the bundle bloated and the win unrealized. Shipping no upgrade message leaves affected users staring at a broken page with no explanation.
Related Resources
Use progressive enhancement and responsive web design to degrade gracefully, performance budgets to capture the bundle win, and feature flags to stage the change. Keep WCAG 2.2 conformance on the new baseline.