Bug 192950 – Implement imagesrcset and imagesizes attributes on link rel=preload

• Status: RESOLVED
• Resolution: FIXED
• Component: DOM

[Dominic Farolino]

It is useful for the UA to be able to preload responsive images. The idea is to allow this with the imagesrcset and imagesizes attributes on <link rel=preload as=image>. Spec PRs and discussions:

 - https://github.com/w3c/preload/issues/120 (Initial discussion)
 - https://github.com/whatwg/html/pull/4048 (HTML Standard PR)
 - https://github.com/w3c/preload/pull/134 (Preload Spec PR (minor))

[Rob Buis]

Created attachment 366072 [details]
Patch

[EWS Watchlist]

Comment on attachment 366072 [details]
Patch

Attachment 366072 [details] did not pass ios-sim-ews (ios-simulator-wk2):
Output: https://webkit-queues.webkit.org/results/11684853

New failing tests:
imported/w3c/web-platform-tests/preload/link-header-preload-srcset.tentative.html

[EWS Watchlist]

Created attachment 366082 [details]
Archive of layout-test-results from ews122 for ios-simulator-wk2

The attached test failures were seen while running run-webkit-tests on the ios-sim-ews.
Bot: ews122  Port: ios-simulator-wk2  Platform: Mac OS X 10.13.6

[Rob Buis]

Created attachment 366276 [details]
Patch

[EWS Watchlist]

Comment on attachment 366276 [details]
Patch

Attachment 366276 [details] did not pass mac-ews (mac):
Output: https://webkit-queues.webkit.org/results/11703692

New failing tests:
imported/w3c/web-platform-tests/preload/link-header-preload-srcset.tentative.html

[EWS Watchlist]

Created attachment 366282 [details]
Archive of layout-test-results from ews100 for mac-highsierra

The attached test failures were seen while running run-webkit-tests on the mac-ews.
Bot: ews100  Port: mac-highsierra  Platform: Mac OS X 10.13.6

[EWS Watchlist]

Comment on attachment 366276 [details]
Patch

Attachment 366276 [details] did not pass mac-wk2-ews (mac-wk2):
Output: https://webkit-queues.webkit.org/results/11703728

New failing tests:
imported/w3c/web-platform-tests/preload/link-header-preload-srcset.tentative.html

[EWS Watchlist]

Created attachment 366283 [details]
Archive of layout-test-results from ews104 for mac-highsierra-wk2

The attached test failures were seen while running run-webkit-tests on the mac-wk2-ews.
Bot: ews104  Port: mac-highsierra-wk2  Platform: Mac OS X 10.13.6

[EWS Watchlist]

Comment on attachment 366276 [details]
Patch

Attachment 366276 [details] did not pass mac-debug-ews (mac):
Output: https://webkit-queues.webkit.org/results/11703770

New failing tests:
imported/w3c/web-platform-tests/preload/link-header-preload-srcset.tentative.html

[EWS Watchlist]

Created attachment 366285 [details]
Archive of layout-test-results from ews112 for mac-highsierra

The attached test failures were seen while running run-webkit-tests on the mac-debug-ews.
Bot: ews112  Port: mac-highsierra  Platform: Mac OS X 10.13.6

[Rob Buis]

Created attachment 366308 [details]
Patch

[Rob Buis]

(In reply to Rob Buis from comment #11)
> Created attachment 366308 [details]
> Patch

Note that the parameter list for loadLink is quite long, it may make sense to consolidate them in a single class, but I think it should be a separate patch.

[Rob Buis]

Created attachment 367724 [details]
Patch

[EWS Watchlist]

Comment on attachment 367724 [details]
Patch

Attachment 367724 [details] did not pass ios-sim-ews (ios-simulator-wk2):
Output: https://webkit-queues.webkit.org/results/11913509

New failing tests:
imported/w3c/web-platform-tests/preload/link-header-preload-srcset.tentative.html

[EWS Watchlist]

Created attachment 367725 [details]
Archive of layout-test-results from ews126 for ios-simulator-wk2

The attached test failures were seen while running run-webkit-tests on the ios-sim-ews.
Bot: ews126  Port: ios-simulator-wk2  Platform: Mac OS X 10.13.6

[Rob Buis]

Would it be better to have a runtime flag for this patch?

[Rob Buis]

Note that the ios-sim failure is likely because of different viewport sizes compared to the other platforms. I'll see if I can change the test or add correct expectation tomorrow.

[youenn fablet]

Comment on attachment 367724 [details]
Patch

Patch looks good to me overall.
Has this shipped in other browsers?

View in context: https://bugs.webkit.org/attachment.cgi?id=367724&action=review

> Source/WebCore/loader/LinkHeader.cpp:290
>      case LinkParameterMedia:

LinkHeader::setValue should probably be String&&

> Source/WebCore/loader/LinkHeader.cpp:297
> +        m_imageSizes = value;

move value here and above

> Source/WebCore/loader/LinkLoader.h:53
> +    bool loadLink(const LinkRelAttribute&, const URL&, const String& as, const String& media, const String& type, const String& crossOrigin, const String&, const String&, Document&);

Maybe give the names here for readability.

> LayoutTests/imported/w3c/web-platform-tests/preload/link-header-preload-nonce.html:15
> +        }, 5000);

This test will take 5 seconds to load. Some will take 3 seconds or 5 seconds.
Is there a way we can run the test faster?
Like have a Link preload in the HTML and do these validation checks once its onload event handler is called.
Or loop until all validation checks pass (and exit after a given number of iterations instead of timing out).

> LayoutTests/imported/w3c/web-platform-tests/preload/link-header-preload-srcset.tentative-expected.txt:3
> +CONSOLE MESSAGE: The resource http://localhost:8800/preload/resources/square.png?from-header&300 was preloaded using link preload but not used within a few seconds from the window's load event. Please make sure it wasn't preloaded for nothing.

Is there a possibility such messages will make the tests flaky?
Should we use DumpJSConsoleLogInStdErr?
Ditto for other tests.

[Rob Buis]

Created attachment 369880 [details]
Patch

[Rob Buis]

Comment on attachment 367724 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=367724&action=review

>> Source/WebCore/loader/LinkHeader.cpp:290
>>      case LinkParameterMedia:
> 
> LinkHeader::setValue should probably be String&&

Done.

>> Source/WebCore/loader/LinkHeader.cpp:297
>> +        m_imageSizes = value;
> 
> move value here and above

Done.

>> Source/WebCore/loader/LinkLoader.h:53
>> +    bool loadLink(const LinkRelAttribute&, const URL&, const String& as, const String& media, const String& type, const String& crossOrigin, const String&, const String&, Document&);
> 
> Maybe give the names here for readability.

Done.

>> LayoutTests/imported/w3c/web-platform-tests/preload/link-header-preload-nonce.html:15
>> +        }, 5000);
> 
> This test will take 5 seconds to load. Some will take 3 seconds or 5 seconds.
> Is there a way we can run the test faster?
> Like have a Link preload in the HTML and do these validation checks once its onload event handler is called.
> Or loop until all validation checks pass (and exit after a given number of iterations instead of timing out).

Isn't it the case that a Link preload in the HTML would not be guaranteed to finish after the Link headers?

For now I am experimenting with iterations as you can see in the updated link-header-preload-nonce.html. However note that in the FAIL case this could still be a similar delay equal to the time out, i.e I do 10 loops of 500ms timeouts so in FAIL case it still takes 5s. link-header-preload-nonce.html is such a FAIL, I have not looked into why, but I think this patch is not the right one since it may be csp related.

>> LayoutTests/imported/w3c/web-platform-tests/preload/link-header-preload-srcset.tentative-expected.txt:3
>> +CONSOLE MESSAGE: The resource http://localhost:8800/preload/resources/square.png?from-header&300 was preloaded using link preload but not used within a few seconds from the window's load event. Please make sure it wasn't preloaded for nothing.
> 
> Is there a possibility such messages will make the tests flaky?
> Should we use DumpJSConsoleLogInStdErr?
> Ditto for other tests.

It is possible with iterations/shorted timeout these messages will go away. FWIW I did not see DumpJSConsoleLogInStdErr being used in wpt so far.

[Rob Buis]

(In reply to youenn fablet from comment #18)
> Comment on attachment 367724 [details]
> Patch
> 
> Has this shipped in other browsers?

It has shipped in Chrome:
https://chromestatus.com/features/5164259990306816

It seems Firefox has no objections but needs to implement rel=preload first:
https://github.com/mozilla/standards-positions/issues/130
https://bugzilla.mozilla.org/show_bug.cgi?id=1515760

[EWS Watchlist]

Comment on attachment 369880 [details]
Patch

Attachment 369880 [details] did not pass ios-sim-ews (ios-simulator-wk2):
Output: https://webkit-queues.webkit.org/results/12190737

New failing tests:
imported/w3c/web-platform-tests/preload/link-header-preload-srcset.tentative.html

[EWS Watchlist]

Created attachment 369896 [details]
Archive of layout-test-results from ews122 for ios-simulator-wk2

The attached test failures were seen while running run-webkit-tests on the ios-sim-ews.
Bot: ews122  Port: ios-simulator-wk2  Platform: Mac OS X 10.14.4

[Frédéric Wang (:fredw)]

Comment on attachment 369880 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=369880&action=review

> Source/WebCore/html/HTMLLinkElement.cpp:265
> +    if (!m_linkLoader.loadLink(m_relAttribute, url, attributeWithoutSynchronization(asAttr), attributeWithoutSynchronization(mediaAttr), attributeWithoutSynchronization(typeAttr), attributeWithoutSynchronization(crossoriginAttr), attributeWithoutSynchronization(imagesrcsetAttr),  attributeWithoutSynchronization(imagesizesAttr), document()))

The loadLink lines are becoming super long. I thought there was a coding style guideline to avoid that, but I only found https://webkit.org/code-style-guidelines/#indentation-wrap-bool-op

[Rob Buis]

Created attachment 369947 [details]
Patch

[EWS Watchlist]

Comment on attachment 369947 [details]
Patch

Attachment 369947 [details] did not pass ios-sim-ews (ios-simulator-wk2):
Output: https://webkit-queues.webkit.org/results/12197067

New failing tests:
imported/w3c/web-platform-tests/preload/link-header-preload-imagesrcset.html

[EWS Watchlist]

Created attachment 369952 [details]
Archive of layout-test-results from ews126 for ios-simulator-wk2

The attached test failures were seen while running run-webkit-tests on the ios-sim-ews.
Bot: ews126  Port: ios-simulator-wk2  Platform: Mac OS X 10.14.4

[Rob Buis]

Created attachment 369976 [details]
Patch

[EWS Watchlist]

Comment on attachment 369976 [details]
Patch

Attachment 369976 [details] did not pass win-ews (win):
Output: https://webkit-queues.webkit.org/results/12199967

New failing tests:
js/dom/custom-constructors.html

[EWS Watchlist]

Created attachment 369989 [details]
Archive of layout-test-results from ews213 for win-future

The attached test failures were seen while running run-webkit-tests on the win-ews.
Bot: ews213  Port: win-future  Platform: CYGWIN_NT-10.0-17763-3.0.5-338.x86_64-x86_64-64bit

[youenn fablet]

Comment on attachment 369976 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=369976&action=review

> Source/WebCore/loader/LinkLoader.cpp:250
> +        url = href;

s/url = href/url = document.completeURL(href)/

> Source/WebCore/loader/LinkLoader.cpp:253
> +        document.addConsoleMessage(MessageSource::Other, MessageLevel::Error, "<link rel=preload> has an invalid `href` value"_s);

This is a bit misleading since we are talking in the message of href, while it might be related to imageSrcSet value.
Can we improve on this, either by adding the URL that is invalid or by making the message more specific.

> Source/WebCore/loader/LinkLoader.cpp:262
> +    auto linkRequest = createPotentialAccessControlRequest(document.completeURL(url), document, crossOriginMode, WTFMove(options));

s/document.completeURL(url)/url

> LayoutTests/ChangeLog:11
> +        * platform/ios-simulator-12-wk2/imported/w3c/web-platform-tests/preload/link-header-preload-imagesrcset-expected.txt: Added.

It should probably be ios-simulator-wk2 instead of ios-simulator-12-wk2.
Is there a way to modify the test to handle various dimensions?

[Rob Buis]

Created attachment 370026 [details]
Patch

[EWS Watchlist]

Comment on attachment 370026 [details]
Patch

Attachment 370026 [details] did not pass win-ews (win):
Output: https://webkit-queues.webkit.org/results/12205022

New failing tests:
js/dom/custom-constructors.html

[EWS Watchlist]

Created attachment 370030 [details]
Archive of layout-test-results from ews210 for win-future

The attached test failures were seen while running run-webkit-tests on the win-ews.
Bot: ews210  Port: win-future  Platform: CYGWIN_NT-10.0-17763-3.0.5-338.x86_64-x86_64-64bit

[EWS Watchlist]

Comment on attachment 370026 [details]
Patch

Attachment 370026 [details] did not pass ios-sim-ews (ios-simulator-wk2):
Output: https://webkit-queues.webkit.org/results/12204984

New failing tests:
imported/w3c/web-platform-tests/preload/link-header-preload-imagesrcset.html

[EWS Watchlist]

Created attachment 370031 [details]
Archive of layout-test-results from ews126 for ios-simulator-wk2

The attached test failures were seen while running run-webkit-tests on the ios-sim-ews.
Bot: ews126  Port: ios-simulator-wk2  Platform: Mac OS X 10.14.4

[Rob Buis]

Created attachment 370034 [details]
Patch

[Rob Buis]

Created attachment 370053 [details]
Patch

[EWS Watchlist]

Comment on attachment 370053 [details]
Patch

Attachment 370053 [details] did not pass ios-sim-ews (ios-simulator-wk2):
Output: https://webkit-queues.webkit.org/results/12209105

New failing tests:
imported/w3c/web-platform-tests/preload/dynamic-adding-preload-imagesrcset.html

[EWS Watchlist]

Created attachment 370068 [details]
Archive of layout-test-results from ews126 for ios-simulator-wk2

The attached test failures were seen while running run-webkit-tests on the ios-sim-ews.
Bot: ews126  Port: ios-simulator-wk2  Platform: Mac OS X 10.14.5

[Rob Buis]

Created attachment 370109 [details]
Patch

[Rob Buis]

Created attachment 370119 [details]
Patch

[EWS Watchlist]

Comment on attachment 370119 [details]
Patch

Attachment 370119 [details] did not pass mac-ews (mac):
Output: https://webkit-queues.webkit.org/results/12215917

New failing tests:
imported/w3c/web-platform-tests/preload/single-download-preload.html

[EWS Watchlist]

Created attachment 370123 [details]
Archive of layout-test-results from ews100 for mac-highsierra

The attached test failures were seen while running run-webkit-tests on the mac-ews.
Bot: ews100  Port: mac-highsierra  Platform: Mac OS X 10.13.6

[Rob Buis]

Created attachment 370124 [details]
Patch

[Rob Buis]

Created attachment 370127 [details]
Patch

[Rob Buis]

Created attachment 370130 [details]
Patch

[Rob Buis]

Comment on attachment 369976 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=369976&action=review

>> Source/WebCore/loader/LinkLoader.cpp:250
>> +        url = href;
> 
> s/url = href/url = document.completeURL(href)/

Done.

>> Source/WebCore/loader/LinkLoader.cpp:253
>> +        document.addConsoleMessage(MessageSource::Other, MessageLevel::Error, "<link rel=preload> has an invalid `href` value"_s);
> 
> This is a bit misleading since we are talking in the message of href, while it might be related to imageSrcSet value.
> Can we improve on this, either by adding the URL that is invalid or by making the message more specific.

Done.

>> Source/WebCore/loader/LinkLoader.cpp:262
>> +    auto linkRequest = createPotentialAccessControlRequest(document.completeURL(url), document, crossOriginMode, WTFMove(options));
> 
> s/document.completeURL(url)/url

Done.

>> LayoutTests/ChangeLog:11
>> +        * platform/ios-simulator-12-wk2/imported/w3c/web-platform-tests/preload/link-header-preload-imagesrcset-expected.txt: Added.
> 
> It should probably be ios-simulator-wk2 instead of ios-simulator-12-wk2.
> Is there a way to modify the test to handle various dimensions?

Unfortunately I did not see a way to modify the test.

[WebKit Commit Bot]

Comment on attachment 370130 [details]
Patch

Clearing flags on attachment: 370130

Committed r245475: <https://trac.webkit.org/changeset/245475>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.

[Radar WebKit Bug Importer]

<rdar://problem/50904042>

[Truitt Savell]

The newly imported imported/w3c/web-platform-tests/preload/link-header-preload-delay-onload.html from https://trac.webkit.org/changeset/245475/webkit

is flakey

History:
https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=imported%2Fw3c%2Fweb-platform-tests%2Fpreload%2Flink-header-preload-delay-onload.html

Diff:
--- /Volumes/Data/slave/highsierra-release-tests-wk1/build/layout-test-results/imported/w3c/web-platform-tests/preload/link-header-preload-delay-onload-expected.txt
+++ /Volumes/Data/slave/highsierra-release-tests-wk1/build/layout-test-results/imported/w3c/web-platform-tests/preload/link-header-preload-delay-onload-actual.txt
@@ -1,4 +1,4 @@
 
 
-PASS Makes sure that Link headers preload resources and block window.onload after resource discovery 
+FAIL Makes sure that Link headers preload resources and block window.onload after resource discovery assert_true: expected true got false

[Rob Buis]

(In reply to Truitt Savell from comment #52)
> -PASS Makes sure that Link headers preload resources and block window.onload
> after resource discovery 
> +FAIL Makes sure that Link headers preload resources and block window.onload
> after resource discovery assert_true: expected true got false

It is probably best to skip this test and make a bug to fix the test result. I should have some time to look into it end of the week.

[Jon Lee]

(In reply to Rob Buis from comment #53)
> (In reply to Truitt Savell from comment #52)
> > -PASS Makes sure that Link headers preload resources and block window.onload
> > after resource discovery 
> > +FAIL Makes sure that Link headers preload resources and block window.onload
> > after resource discovery assert_true: expected true got false
> 
> It is probably best to skip this test and make a bug to fix the test result.
> I should have some time to look into it end of the week.

Can we wrap this in an experimental feature flag?

[Alexey Proskuryakov]

Let's roll back at this point. The test is still flaky, and yes, features need flags.

[Shawn Roberts]

Reverted r245475 for reason:

Newly imported test is flaky. Features need flags.

Committed r245825: <https://trac.webkit.org/changeset/245825>

[Rob Buis]

Created attachment 371044 [details]
Patch

[Rob Buis]

Created attachment 371059 [details]
Patch

[Rob Buis]

Created attachment 371070 [details]
Patch

[Rob Buis]

Created attachment 371073 [details]
Patch

[Rob Buis]

Comment on attachment 371073 [details]
Patch

Sorry for not including the runtime flag in the patch that was reverted! I added it in the latest patch. I also removed the flaky test, since it is not actually testing imagesrcset/imagesizes.

[youenn fablet]

Comment on attachment 371073 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=371073&action=review

> Source/WebCore/loader/LinkLoader.cpp:245
> +    if (type == CachedResource::Type::ImageResource && !imageSrcSet.isEmpty()) {

We probably need to add a runtime check here to disable the behavior if the flag is not enabled.

> Source/WebCore/loader/LinkLoader.h:66
> +    static std::unique_ptr<LinkPreloadResourceClient> preloadIfNeeded(const LinkRelAttribute&, const URL& href, Document&, const String& as, const String& media, const String& type, const String& crossOriginMode, const String&, const String&, LinkLoader*);

Could we add the names of the string variables to improve readability.

> LayoutTests/imported/w3c/ChangeLog:66
> +

Not needed.

> LayoutTests/platform/ios-simulator-12-wk2/imported/w3c/web-platform-tests/preload/dynamic-adding-preload-imagesrcset-expected.txt:2
> +FAIL Makes sure that a dynamically added preload with imagesrcset works assert_equals: resources/square.png?400 expected 1 but got 0

Could we add specific variations of these tests for iOS?

[Rob Buis]

Created attachment 371145 [details]
Patch

[EWS Watchlist]

Comment on attachment 371145 [details]
Patch

Attachment 371145 [details] did not pass mac-ews (mac):
Output: https://webkit-queues.webkit.org/results/12355653

New failing tests:
inspector/canvas/recording.html

[EWS Watchlist]

Created attachment 371148 [details]
Archive of layout-test-results from ews102 for mac-highsierra

The attached test failures were seen while running run-webkit-tests on the mac-ews.
Bot: ews102  Port: mac-highsierra  Platform: Mac OS X 10.13.6

[Rob Buis]

Created attachment 371149 [details]
Patch

[EWS Watchlist]

Comment on attachment 371149 [details]
Patch

Attachment 371149 [details] did not pass mac-wk2-ews (mac-wk2):
Output: https://webkit-queues.webkit.org/results/12356219

New failing tests:
http/wpt/service-workers/service-worker-networkprocess-crash.html

[EWS Watchlist]

Created attachment 371156 [details]
Archive of layout-test-results from ews104 for mac-highsierra-wk2

The attached test failures were seen while running run-webkit-tests on the mac-wk2-ews.
Bot: ews104  Port: mac-highsierra-wk2  Platform: Mac OS X 10.13.6

[Rob Buis]

Created attachment 371157 [details]
Patch

[EWS Watchlist]

Comment on attachment 371157 [details]
Patch

Attachment 371157 [details] did not pass mac-ews (mac):
Output: https://webkit-queues.webkit.org/results/12356774

New failing tests:
inspector/canvas/recording.html

[EWS Watchlist]

Created attachment 371159 [details]
Archive of layout-test-results from ews101 for mac-highsierra

The attached test failures were seen while running run-webkit-tests on the mac-ews.
Bot: ews101  Port: mac-highsierra  Platform: Mac OS X 10.13.6

[Rob Buis]

Comment on attachment 371073 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=371073&action=review

>> Source/WebCore/loader/LinkLoader.cpp:245
>> +    if (type == CachedResource::Type::ImageResource && !imageSrcSet.isEmpty()) {
> 
> We probably need to add a runtime check here to disable the behavior if the flag is not enabled.

Done.

>> Source/WebCore/loader/LinkLoader.h:66
>> +    static std::unique_ptr<LinkPreloadResourceClient> preloadIfNeeded(const LinkRelAttribute&, const URL& href, Document&, const String& as, const String& media, const String& type, const String& crossOriginMode, const String&, const String&, LinkLoader*);
> 
> Could we add the names of the string variables to improve readability.

Done.

>> LayoutTests/imported/w3c/ChangeLog:66
>> +
> 
> Not needed.

Removed.

>> LayoutTests/platform/ios-simulator-12-wk2/imported/w3c/web-platform-tests/preload/dynamic-adding-preload-imagesrcset-expected.txt:2
>> +FAIL Makes sure that a dynamically added preload with imagesrcset works assert_equals: resources/square.png?400 expected 1 but got 0
> 
> Could we add specific variations of these tests for iOS?

I still don't know how to do that. If anybody has an idea, I can do a follow-up patch.

[Rob Buis]

Created attachment 371161 [details]
Patch

[WebKit Commit Bot]

Comment on attachment 371161 [details]
Patch

Clearing flags on attachment: 371161

Committed r246045: <https://trac.webkit.org/changeset/246045>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.

[youenn fablet]

> >> LayoutTests/platform/ios-simulator-12-wk2/imported/w3c/web-platform-tests/preload/dynamic-adding-preload-imagesrcset-expected.txt:2
> >> +FAIL Makes sure that a dynamically added preload with imagesrcset works assert_equals: resources/square.png?400 expected 1 but got 0
> > 
> > Could we add specific variations of these tests for iOS?
> 
> I still don't know how to do that. If anybody has an idea, I can do a
> follow-up patch.

IIUC the issue, the viewport for iOS simulator is different, which will trigger preloading of different resources than the test expects.
I would copy the test in some LayoutTests iOS specific folder and update the test so that we validate preloading with the iOS simulator viewport works.

Could the test get the viewport and from it, compute on its own which preload should be selected?

Alternatively, the test could be copie-then-made-iOS-specific.

[Rob Buis]

(In reply to youenn fablet from comment #76)
> > >> LayoutTests/platform/ios-simulator-12-wk2/imported/w3c/web-platform-tests/preload/dynamic-adding-preload-imagesrcset-expected.txt:2
> > >> +FAIL Makes sure that a dynamically added preload with imagesrcset works assert_equals: resources/square.png?400 expected 1 but got 0
> > > 
> > > Could we add specific variations of these tests for iOS?
> > 
> > I still don't know how to do that. If anybody has an idea, I can do a
> > follow-up patch.
> 
> IIUC the issue, the viewport for iOS simulator is different, which will
> trigger preloading of different resources than the test expects.
> I would copy the test in some LayoutTests iOS specific folder and update the
> test so that we validate preloading with the iOS simulator viewport works.
> 
> Could the test get the viewport and from it, compute on its own which
> preload should be selected?
> 
> Alternatively, the test could be copie-then-made-iOS-specific.

Ok, I will have a look tomorrow.

[Rob Buis]

(In reply to Rob Buis from comment #77)
> (In reply to youenn fablet from comment #76)
> > IIUC the issue, the viewport for iOS simulator is different, which will
> > trigger preloading of different resources than the test expects.
> > I would copy the test in some LayoutTests iOS specific folder and update the
> > test so that we validate preloading with the iOS simulator viewport works.
> > 
> > Could the test get the viewport and from it, compute on its own which
> > preload should be selected?
> > 
> > Alternatively, the test could be copie-then-made-iOS-specific.
> 
> Ok, I will have a look tomorrow.

I fixed the tests in the WPT repo and imported them here:
https://bugs.webkit.org/show_bug.cgi?id=198533