Thanks
2026년 10월 6일 화요일 오후 7시 1분 28초 UTC+9에 [email protected]님이
작성:
On 10/5/26 11:24 p.m., Chromestatus wrote:
*Contact emails*
[email protected]
*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
<https://issues.chromium.org/issues?q=customfield1222907:%22Blink%3ENetwork%22>
*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.
From what I can tell this change is just scoped to resource
timing, so only a handful of RUM providers would be affected (in
the hypothetical that some script is expecting
"application/atom+xml" via PerformanceResourceTiming.contentType
but now seeing "application/xml" and exploding). Is there some
issue where RUM providers are asking for this / might find out
about the change?
Even so, I'd love to better understand the compat risk of this
proposed change. Above it mentions "Enterprise web applications
that rely on previous `contentType` values may observe behavior
changes" - do we think enterprises are more likely to be affected
by this change? Are we able to estimate or reason about breakage
for non-enterprise use cases? e.g., some site is expecting
"application/atom+xml" via PerformanceResourceTiming.contentType
but now seeing "application/xml" going to break something (that
sounds far-fetched, I dunno)?
As an aside,
https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/actor/execution_engine.cc;l=288-299
should probably be looked at by the security reviewer/other owners
for this change. "image/svg+xml" will start to return true here
with this change, when it return false before, if I'm reading the
code correctly.
I see in your CL
<https://chromium-review.git.corp.google.com/c/chromium/src/+/8301932/comments/dabacf37_52e1d902>
you wrote "MIMETypeRegistry::IsXMLMIMEType unification is
intentionally left as follow-up" - can you explain your plan
there? That seems like a riskier change to me, since it will
affect navigation & loading.
/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.
We should probably request a formal position at
https://github.com/mozilla/standards-positions, because the ideal
outcome is all browsers are doing the same thing rather than
diverging in subtle ways.
/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.
Ditto here - https://github.com/WebKit/standards-positions please.
/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
<https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_platform_tests.md>?*
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
<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 (e.g. links to
known github issues in the project for the feature specification)
whose resolution may introduce web compat/interop risk (e.g.,
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
<https://chromestatus.com>.
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