Blink forked from Webkit more than a decade ago. There are tests that try to measure their compatibility, like https://wpt.fyi/results/?label=experimental&label=master&ali... (part of https://web-platform-tests.org/). They also have different JS engines these days (V8 vs JSCore).

That said, as a frontend dev, I very very rarely run into real world issues between Safari and Chrome. More often it's Firefox/Gecko that lags behind, usually in some sort of graphics optimization (SVG and canvas stuff). Also some differences in WebGL and webgpu support, but those are niche enough that it's not a big deal yet.