HTML Validator
Validate HTML and get the exact line and cause of every error.
Input
Output
Ready
Drop a file to load it
About this tool
Check that an HTML document is well-formed, with errors reported by line and column and explained rather than just named.
When you would use it
- Catching a malformed HTML document before it reaches production
- Finding the exact position of a syntax error
- Checking a file a tool rejected
Worked example
<div class="card"> <h2>Ada Lovelace</h2> <p>Score: <strong>91.5</strong></p> </div>
Valid · 4 elements · 3 maxDepth · 0 ids · 0 warnings · 0 errors
Things worth knowing
- HTML parsing is forgiving by design, so structural problems are reported as errors and everything else as warnings.
- Accessibility issues — missing
alttext, unlabelled inputs, duplicate ids — are surfaced as warnings.
Questions
Frequently asked
What does "well-formed" mean for HTML?
Tags are balanced and nest correctly. HTML parsers recover from most mistakes, so this also reports the problems a browser would silently paper over.
Is my data uploaded anywhere?
No. Every tool on JSONWorks runs entirely in your browser using JavaScript. Your input is never sent to a server, never logged and never stored. The simplest proof is to disconnect from the internet — the tools keep working. The site does load Google Fonts and a Google Analytics page-view tag, so you will see those two requests in your network tab, but neither receives anything you type or paste.
Does it work offline?
Yes. JSONWorks is installable as a progressive web app and caches itself on first visit, so every tool keeps working without a connection.
Related