Compatibility test cases, past does it look right

Compatibility testing is usually understood as looking for visual differences, which is the least valuable part of it. The defects that matter break functionality, and the largest group comes from privacy configurations rather than from rendering engines. A storage accessor that throws instead of returning null in a private window will crash a page at load if the read is not wrapped. Third party cookies blocked by default break embedded payment frames and identity redirects, and they break them silently, leaving an iframe that never resolves and a console with nothing in it.

Data handling differs between engines in ways that produce a defect on exactly one browser. Date parsing is the classic: a non standard format string is accepted by some engines and returns an invalid date in others, so a feature works everywhere except Safari and the bug report is impossible to reproduce locally. Locale is the companion case, because a browser configured for a comma decimal separator will both render and parse numbers differently, and any code assuming a full stop will silently mangle user input.

Mobile is where emulation stops being sufficient. Viewport height units behave differently across mobile engines depending on browser chrome and keyboard state, so a full height layout that is perfect in a desktop responsive view puts the focused input underneath the keyboard on a real phone. In app browsers inside social and messaging apps are a large share of mobile traffic and impose their own storage and pop up restrictions, which means sign in and payment need testing specifically inside them rather than only in the standalone browser.

Finally, the environment around the page is part of the test. Ad and tracker blockers are widely used, so any journey that waits for an analytics or tag manager script to load will fail for a substantial minority of users, and the failure is invisible in analytics precisely because analytics is what is blocked. Blocked pop ups break authentication flows that open a window. And a deployment while a browser holds cached assets can leave a stale bundle calling a changed API, which is why detecting an outdated client and prompting a reload belongs in this set rather than in a release checklist.