HTML Tester
Test HTML for unclosed or mismatched tags with a live, sandboxed preview.
Every tag looks properly opened and closed.
Checks that tags are balanced and renders a live, sandboxed preview that never executes scripts — pasting HTML from an untrusted source is safe. This is a structural check, not a full HTML5 spec validator. Everything runs in your browser; nothing you type is sent anywhere.
Need the opposite? Use HTML Viewer
What is HTML Tester?
Paste HTML and this tool checks that every tag is properly opened and closed, flags any mismatches, and renders a live preview in a sandboxed frame that never executes scripts — useful for quickly spotting a stray unclosed <div> before it breaks a page's layout.
FAQ
No — this is a structural balance check (are tags properly opened and closed), not a full spec validator. It won't catch every possible HTML validity issue.
Yes — the preview renders in a sandboxed iframe with no script execution allowed, and nothing you type is sent anywhere.