No Deprecations.

Deprecated APIs may break in future browser versions

What does this check test?

This check identifies the use of deprecated web APIs, CSS properties, and JavaScript features that browsers have marked for removal. Deprecated APIs continue to work in current browser versions but will be removed in future updates, potentially breaking your site without warning. Common deprecations include `document.write()`, `unload` event handlers, third-party cookies in non-partitioned contexts, `AppCache`, and various prefixed CSS properties. Chrome DevTools Issues panel and Console panel log deprecation warnings with links to migration guides.

Why does it matter?

Using deprecated APIs creates a ticking time bomb — your site works today but may break in a future browser update without any code change on your end. Browser vendors provide deprecation warnings well in advance (typically 6-12 months), giving developers time to migrate. Deprecated APIs are often replaced because they have performance problems (e.g., `document.write()` blocks parsing and can delay page load by seconds), security issues (e.g., third-party cookies enable cross-site tracking), or better alternatives exist. Migrating away from deprecated APIs proactively is much cheaper than emergency fixes when they are removed.

Who is affected?

Legacy codebases that have not been actively maintained, sites using old jQuery plugins that rely on deprecated browser APIs, sites using polyfills for APIs that have changed specification, and sites with old analytics or advertising scripts are most commonly affected. Enterprise applications with long update cycles are particularly vulnerable because deprecated APIs may be removed before the next scheduled update.

Where does this apply?

Chrome DevTools Issues panel (Ctrl+Shift+I > Issues tab) lists all deprecation warnings with detailed descriptions and affected code locations. The Console panel also logs deprecation warnings, often prefixed with '[Deprecation]'. Lighthouse flags deprecated APIs in the 'Uses deprecated APIs' diagnostic. Common locations: inline `<script>` tags using `document.write()`, event handlers using the `unload` event, CSS using `-webkit-` prefixed properties that have unprefixed versions, and old API patterns like synchronous XMLHttpRequest.

How to fix it

// Deprecated
document.write('<script src="widget.js"><\/script>');

// Modern alternative
const script = document.createElement('script');
script.src = 'widget.js';
document.head.appendChild(script);

// Deprecated — unreliable and blocks bfcache
window.addEventListener('unload', saveState);

// Modern alternative
document.addEventListener('visibilitychange', () => {
  if (document.visibilityState === 'hidden') {
    saveState();
  }
});

References

• Uses deprecated APIs — Chrome Developers
• Chrome Deprecations — Chrome Platform Status
• Browser compatibility — MDN