Why a page works on your computer but not online
When you double-click an .html file, the browser loads it from your disk with very forgiving rules: it can reach any file on your computer, nothing is case-sensitive on Windows or macOS, and there is no HTTPS to enforce. Put the same file on a web server and three things change at once — paths are resolved against a URL, file names become case-sensitive, and an https:// page refuses to load anything over plain http://. This checker looks for exactly those differences, plus the missing head tags that make a page look wrong on a phone.
What each check means
| Finding | What breaks | Fix |
|---|---|---|
Local paths (file://, C:\, /Users/) | Images, CSS or scripts never load for anyone else | Use relative paths and upload the files together |
| Backslashes in paths | 404s on every web server | Replace \ with / |
http:// resources | Blocked as mixed content on HTTPS | Switch to https:// |
| No viewport meta | Page renders tiny on phones | Add the viewport tag |
| No charset | Garbled accents and emoji | Add <meta charset="UTF-8"> |
| Unclosed tags | Layout collapses or content disappears | Close the tag where the block ends |
| Duplicate IDs | JavaScript targets the wrong element | Make every id unique |
Checking a whole folder or ZIP
This page checks one HTML document. For a multi-file site, the ZIP site checker opens the whole archive in your browser and verifies that every referenced file actually exists, with the exact case. The troubleshooting guide goes through each failure in depth.