Until ~2010ish, Opera was per-se pretty much competitive (after that, the other vendors were implementing new specs quicker), but there were some notable differences, especially around places where specs allowed multiple behaviours and every vendor except for Opera had converged on one behaviour a decade earlier (rounding of CSS values comes to mind here, but was far from the only case), and that caused more and more problems and web-developer pain as the web became increasingly complex.

Iirc, Opera was actually quite ahead of Firefox in passing acid3, though can't remember if they were first or if WebKit was

It probably helped that Håkon Lie, who proposed/invented CSS, worked at Opera.