IconVault's CORS Preflight Flow walks you through exactly what the browser checks before letting JavaScript read a cross-origin response. Pick from 8 realistic scenarios, simple GETs, JSON POSTs, custom headers, credentialed requests and classic failures, and step through the decision algorithm one check at a time until you reach the allow or block verdict. It is free and runs fully in your browser.

CORS errors are confusing because the network tab shows a successful 200 while fetch() still throws. The confusion disappears once you see the algorithm: same-origin check, simple-request check, optional preflight OPTIONS round-trip, then response header validation. Each failing scenario in this tool ends with the concrete server-side fix, so you leave knowing not just what went wrong but how to fix it.