Deprecate Mutation Events

concepts

1. Mutation Events vs MutationObserver

   Run a 500-node insertion through both APIs at once and compare events fired, records seen, and total time spent in each handler.

2. Mutation-event linter for your code

   The migration use case from the Intent to Remove: paste any JS, get a per-line audit of legacy event names and the recommended MutationObserver replacement.

3. MutationObserver recipe book

   Drop-in recipes for each deprecated event — DOMNodeInserted, DOMNodeRemoved, DOMSubtreeModified, DOMAttrModified — with a sandbox you can mutate to watch the observer report records in real time.

4. Migration Wizard

   Select any deprecated Mutation Event and see the exact MutationObserver equivalent side-by-side. A live DOM sandbox lets you fire insert, remove, text, and attribute mutations and watch the replacement observer respond.

5. Performance benchmark

   Run a configurable DOM insertion benchmark (100–2000 nodes) comparing Mutation Events vs MutationObserver. The key insight: Mutation Events fire once per node synchronously; MutationObserver fires once per batch asynchronously. Bar charts, callback counts, and a live event log.

why it shipped

Mutation Events have been deprecated for over a decade, with the replacement (Mutation Observer) available also for over a decade. The fact that these events are still supported in browsers makes the addition of new features much more difficult, prohibitively so in some cases. For example, these feature requests and projects are all negatively impacted by the existence of Mutation Events: - iFrame reparenting: https://github.com/whatwg/html/issues/5484 and https://github.com/whatwg/dom/issues/891 - Child reordering: https://github.com/whatwg/dom/issues/586 - DOM Parts and batch DOM updates:

references

• ChromeStatus entry
• Spec