If I recall correctly, the bug here was that the test contains:

  <meta name="timeout" content="long">

And therefore the timeout should be 30 seconds (by default the regular timeout is 6 seconds, slow is 30 seconds).

File:

https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/external/wpt/html/semantics/tabular-data/processing-model-1/span-limits.html?q=external/wpt/html/semantics/tabular-data/processing-model-1/span-limits.html

But actually in practice it is behaving correctly: for example on WebKit Win10 it's not expected to timeout, and the runs are longer than 6s, and it doesn't timeout.

Flakiness dashboard:

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_tests&tests=external/wpt/html/semantics/tabular-data/processing-model-1/span-limits.html

However it does seem to timeout on debug builds.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a84409be240cbf30cad2467eb4b61c3a46db671b

commit a84409be240cbf30cad2467eb4b61c3a46db671b
Author: Quinten Yearsley <qyearsley@chromium.org>
Date: Wed Oct 25 16:29:08 2017

Change timeout expectation for span-limits.html

Bug:  755810 
Change-Id: I0da4c59e514dd799e24e42997697f27be5b4a96f
Reviewed-on: https://chromium-review.googlesource.com/732285
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511481}
[modify] https://crrev.com/a84409be240cbf30cad2467eb4b61c3a46db671b/third_party/WebKit/LayoutTests/TestExpectations

Works as intended (I think the timeout behavior is right).