there are 2 of the web-platform-test resize-observer/ tests which are expected timeout:
/resize-observer/observe.html
/resize-observer/svg.html

I verified both of these failures locally on windows10, what I see is the first half of the tests pass, but the second half generally fail with this error:
can't access property "inlineSize", entries[0].contentBoxSize[0] is undefined

the last test end with a timeout and this error message:
test18: an observation is fired when device-pixel-content-box is being observed assert_unreached: Caught a throw, possible syntax error Reached unreachable code

_nextStep/<@http://web-platform.test:8000/resize-observer/resources/resizeTestHelper.js:72:25
Test.prototype.step@http://web-platform.test:8000/resources/testharness.js:1977:25
_nextStep@http://web-platform.test:8000/resize-observer/resources/resizeTestHelper.js:71:25
start/<@http://web-platform.test:8000/resize-observer/resources/resizeTestHelper.js:135:12
Test.prototype.step@http://web-platform.test:8000/resources/testharness.js:1977:25
start@http://web-platform.test:8000/resize-observer/resources/resizeTestHelper.js:133:23
test18@http://web-platform.test:8000/resize-observer/observe.html:906:17
@http://web-platform.test:8000/resize-observer/observe.html:933:15
promise callback*@http://web-platform.test:8000/resize-observer/observe.html:933:4

I am not sure if the timeout on the last test is unrelated to inline-size or if it is related to bug 1587973.

:svoisen, as triage owner can you help me find someone to get an answer to determine if these test failures and timeouts are an issue with the test case, a bug in firefox, or something we don't support?

I think this failure is just indicating that we don't support the device-pixel-content-box feature of resize-observer.

This is known (device-pixel-content-box was added to the spec near the end of 2019, after our implementation). I'm not sure we've got a bug on it yet, though. I'll file one and include this as a dependency.

Separately/also: we could perhaps improve these tests to explicitly test for device-pixel-content-box and fail early (rather than timing out with an exception thrown) if it's missing.

I will create a patch to mark these tests as backlog so they will still run on mozilla-central and we can catch failures, but we won't run them per commit to look for active regressions as they are known to timeout (a 3rd test is marked expected fail)

Mark wpt tests that depend on device-pixel-content-box as backlog (tier-2)

https://hg.mozilla.org/mozilla-central/rev/bffa16d830c1