Now, there are just a few differences between the tests with conversion and without:

https://codereview.chromium.org/2485493002

In csswg-test:

(a) Path to support file from reference html file is different for reference tests in csswg-test/css-shapes-1/shape-outside/supported-shapes. So we probably still want the behavior of convert_reference_relpaths in _W3CTestConverter. What we really want to do here is to change the layout test runner to understand <link rel=match href="...">, and then the paths in reference files don't need to be changed.

(b) Conversion adds "-webkit-" prefix to text-emphasis CSS property; this does change behavior, since the unprefixed version "text-emphasis" is only supported by Firefox (-webkit-text-emphasis is supported in Chrome, Opera, Safari).

(c) Conversion mangles "support/tcu-font.woff" into "suppotcu-font.woff". The behavior without conversion is correct, and the test could be failing for us just because of the change when the test is imported.