Inspect the problem before changing code
Reproduce the issue, note the viewport and browser, and reduce the scenario to a few clear steps.
Use the Elements and Styles panels
Inspect computed values, box sizing and layout overlays to locate unexpected margins, hidden elements and responsive breakpoints.
Read the Console carefully
JavaScript errors often identify a file and line number. Fix the first relevant error, then retest because one failure may cause others.
Investigate the Network tab
Check status codes, incorrect resource paths, slow requests and caching. A missing stylesheet or blocked script can make a working page appear broken.
Verify your fix
Test on mobile and desktop, with keyboard input, and on both light and dark theme if available. Record the cause and fix so a similar issue is easier next time.