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Specification https://mimesniff.spec.whatwg.org/#xml-mime-type Summary Chrome is updating how it Detects XML MIME types to fully align with Web standards. Chrome will now recognize valid MIME types whose subtype ends in `+xml` as XML across Chrome and Android WebView, including non-application types such as `text/example+xml` and `image/example+xml`. For these responses, `PerformanceResourceTiming.contentType` returns application/xml. The value for image/svg+xml remains unchanged. Enterprise web applications that rely on previous `contentType` values may observe behavior changes. Chrome Actor also uses this shared XML classification for dangerous MIME type navigation checks, so it may block navigations to additional non-application +xml responses. IT admins should check whether their applications depend on these MIME types or the previous `contentType` values. Blink component Blink>Network Web Feature ID No information provided Motivation Chromium currently applies generic +xml suffix matching only to application/* MIME types, although the WHATWG MIME Sniffing Standard defines any valid MIME type whose subtype ends in +xml as an XML MIME type. As a result, non-application types such as text/example+xml are not minimized to application/xml in PerformanceResourceTiming.contentType. This change aligns Chromium's classification with the existing standard and improves interoperability. Implementation (merged behind an experimental feature): https://chromium-review.googlesource.com/c/chromium/src/+/8301932 Initial public proposal No information provided TAG review Requesting a TAG review exception for this small conformance change to the existing WHATWG MIME Sniffing Standard. The change broadens XML MIME type suffix matching to include non-application types. No new API surface or API design is introduced. TAG review status Not applicable Goals for experimentation None Risks Interoperability and Compatibility This change aligns XML MIME type classification with the existing WHATWG MIME Sniffing Standard. The web-visible change is that PerformanceResourceTiming.contentType returns "application/xml" for additional non-application MIME types whose subtype ends in "+xml". Code that depends on the previous contentType value may observe a different result. The special handling of "image/svg+xml" is preserved. The change is controlled by SpecCompliantXmlMimeTypes, allowing the previous behavior to be restored by disabling the feature. Gecko: No signal Firefox recognizes non-application "+xml" MIME types in its XHR implementation, but the Resource Timing minimization behavior differs. In the WPT PR #62420 run dated 2026-09-04, Firefox Nightly 157.0a1 failed all five added XML cases. For example, contentType returned "text/example+xml" instead of "application/xml". Results: https://wpt.fyi/results/resource-timing/content-type-minimization.html?run_id=6323630396145664 No formal standards position has been requested. An exception is requested for this small conformance change, subject to API owner review. WebKit: No signal WebKit recognizes non-application "+xml" MIME types in its XML MIME classification code. This does not establish support for PerformanceResourceTiming.contentType. In the WPT PR #62420 run dated 2026-09-04, Safari Technology Preview 251 failed all five added XML cases because contentType was undefined. Results: https://wpt.fyi/results/resource-timing/content-type-minimization.html?run_id=5125540410556416 No formal standards position has been requested. An exception is requested for this small conformance change, subject to API owner review. Web developers: No signals Other signals: Ergonomics No new API or calling convention is introduced. Developers continue to read PerformanceResourceTiming.contentType through the existing Resource Timing API. Activation No developer opt-in is required. Applications that rely on the previous contentType value for non-application "+xml" responses may need to adjust their expectations. Security The change broadens the MIME types classified as XML by blink::IsXMLMimeType. It does not change the existing response-access checks that govern exposure of PerformanceResourceTiming.contentType. The shared helper is also used by Chrome Actor's dangerous-MIME-type navigation checks. When those checks and SpecCompliantXmlMimeTypes are enabled, additional non-application "+xml" types may be blocked. WebView application risks Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications? Applications that inspect PerformanceResourceTiming.contentType may observe "application/xml" for additional non-application "+xml" MIME types. Applications that depend on the previous value may behave differently. The change is guarded by SpecCompliantXmlMimeTypes, which provides a way to restore the previous behavior. Debuggability 1. Basic support Manually checked on macOS using Content Shell with --enable-features=SpecCompliantXmlMimeTypes and DevTools attached to the test page. Console evaluation and autocomplete worked during the check. Chrome DevTools for agents was not separately tested. 2. Inspectability A local server returned Content-Type: text/example+xml. The response was fetched and its PerformanceResourceTiming entry was obtained through PerformanceObserver. Reading entry.contentType in the DevTools Console returned "application/xml" as expected. Autocomplete for contentType was also verified. The server's original Content-Type header and the minimized Resource Timing value are distinct. 3. Extended support No dedicated DevTools workflow is proposed. The changed value can be inspected through existing Console functionality. 4. Automated testing Yes. The existing WPT resource-timing/content-type-minimization.html exercises the added non-application "+xml" cases. Implementation: https://chromium-review.googlesource.com/c/chromium/src/+/8301932 WPT export: https://github.com/web-platform-tests/wpt/pull/62420 Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)? Yes The change uses shared Chromium MIME classification code with no platform-specific restrictions. Support is intended for Windows, macOS, Linux, ChromeOS, Android, and Android WebView. Is this feature fully tested by web-platform-tests? Yes The implementation adds test cases to: mimesniff/mime-types/resources/mime-types-minimized.json These cases are exercised by: resource-timing/content-type-minimization.html Coverage includes text/example+xml, image/example+xml, font/example+xml, uppercase MIME types, and MIME parameters. Parameterized C++ unit tests cover both enabled and disabled feature states, existing XML MIME types, additional "+xml" types, and invalid MIME inputs. Implementation and tests: https://chromium-review.googlesource.com/c/chromium/src/+/8301932 WPT export: https://github.com/web-platform-tests/wpt/pull/62420 The WPT additions were merged upstream on September 30, 2026: https://github.com/web-platform-tests/wpt/pull/62420 PR-level browser results are available, including the five added XML cases: https://wpt.fyi/results/resource-timing/content-type-minimization.html?run_id=6323630396145664&run_id=5125540410556416 Flag name on about://flags enable-experimental-web-platform-features Finch feature name SpecCompliantXmlMimeTypes Rollout plan Will ship enabled for all users Requires code in //chrome? False Tracking bug https://issues.chromium.org/issues/362282752 Measurement No dedicated UseCounter is added by this change. The occurrence of affected non-application "+xml" MIME types is not separately measured by this implementation. Availability expectation Expected to be available by default on Blink platforms after launch approval and default enablement of SpecCompliantXmlMimeTypes. Adoption expectation Existing users of PerformanceResourceTiming.contentType will receive spec-compliant values for additional XML MIME types without changing their API usage. No quantitative adoption estimate is available. Adoption plan Communicate the behavior change through the Blink intent process and track any necessary documentation and browser compatibility updates. Non-OSS dependencies Does the feature depend on any code or APIs outside the Chromium open source repository and its open-source dependencies to function? None. The implementation uses Chromium code and its existing open-source dependencies. Estimated milestones Shipping on desktop 158 Shipping on Android 158 Shipping on WebView 158 Anticipated spec changes Open questions about a feature may be a source of future web compat or interop issues. Please list open issues (eg links to known github issues in the project for the feature specification) whose resolution may introduce web compat/interop risk (eg, changing to naming or structure of the API in a non-backward-compatible way). None anticipated. This change implements the existing XML MIME type definition and MIME type minimization rules in the WHATWG MIME Sniffing Standard. Link to entry on the Chrome Platform Status https://chromestatus.com/feature/5092584283308032?gate=6712519581368320 Links to previous Intent discussions Intent to Prototype: https://groups.google.com/a/chromium.org/d/msgid/blink-dev/6a922400.e8a51b51.6abb3.033c.GAE%40google.com This intent message was generated by Chrome Platform Status. -- You received this message because you are subscribed to the Google Groups "blink-dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/a/chromium.org/d/msgid/blink-dev/6ac46a00.082f26cb.22c2b.02d5.GAE%40google.com.
