I am investigating all tests that are expected TIMEOUT with the goal of getting them fixed, or not running by default or at all.

I find these 4 tests are expected TIMEOUT on all platforms:
/css/css-variables/variable-transitions-from-no-value.html
/css/css-variables/variable-transitions-to-no-value.html
/css/css-variables/variable-transitions-transition-property-variable-before-value.html
/css/css-variables/variable-transitions-value-before-transition-property-variable.html

I have verified these fail locally on my windows 10 laptop. These end up with a status of "harness timeout" and "2 tests not run", where we fail on:
Verify CSS variable value after transition

I assume since they all have the same failure pattern that they are related.

:svoisen, can you take a look at these tests and determine if they are supported, broken tests, or a broken product? Feel free to redirect as needed.

This is a bug in our code. bug 1465776.

I am going to make this as a duplicate of bug 1465776.