How to use the HTML Validator
- 1. Paste your HTMLA whole page or just a section.
- 2. Check itThe checker reads every tag and keeps track of what is open.
- 3. Fix the listEach issue shows its line number and type: error, warning or tip.
What this checker looks for
- Structure: tags opened and never closed, closing tags with no opening tag, wrong nesting like
<div><p>…</div>, and closing tags on void elements like </br>. - IDs: the same id used twice, which breaks labels and JavaScript.
- Accessibility: images without alt, inputs without a label, links inside links and empty hrefs.
- Head basics: doctype, lang, title, charset and viewport.
- Good practice: obsolete tags, skipped heading levels, several <h1>s and target="_blank" without rel="noopener".
Tags that HTML allows you to leave open, such as <li>, <p> and <td>, are handled the way browsers handle them.
Worked example and limits
The sample page has deliberate mistakes. The checker finds 2 errors and 4 warnings, plus a tip. The errors: the image on line 9 has no alt, and id="box" is used twice (lines 10 and 11). The warnings: <html> has no lang, there is no charset or viewport meta tag, and <center> is obsolete. The unclosed <p> on line 10 is not flagged, because HTML lets a paragraph end when its parent closes. It also tips you that the headings jump from h1 to h3.
This is a basic checker for the mistakes beginners make most. It is not the official W3C Nu validator, which checks every rule in the HTML standard, such as which attributes each element allows. Use this one for quick fixes while you work, and the W3C validator before you publish.
Frequently asked questions
Why is an unclosed <li> not an error?
HTML lets you leave some tags open, such as
and
Does it check CSS or JavaScript?
No, only HTML. Contents of style and script blocks are skipped.
Is my code uploaded?
No. Everything is checked in your browser.
Is the checker free?
Yes, it is free. Your first report asks for your name and WhatsApp number once, then all Uncodemy tools open without it.