Managed to make Dillo pass a small subset of the Web Platform Tests, which is the reference test suite for modern web browsers.
So far, the CSS2 tests¹ report:
3032 BAD
5359 FAIL
787 OK
Which is not too bad, given the large number of CSS features missing in Dillo. Now it would be easier see what is properly supported and detect regressions.
BAD typically means the test requires JS and FAIL that there is a mismatch with the reference render.