How to use HTML Checker
- Paste your HTML, drop an
.htmlfile onto the editor, or load the Example with mistakes. - The report updates as you type. Each problem is marked Error, Warning or Tip and shows its line and column.
- Click a problem to jump to it in the editor, follow the suggested fix, and watch the score go up.
About this tool
Browsers are forgiving: they quietly repair broken HTML instead of showing an error. That sounds helpful, but it means a missing </div> or an unclosed quote can silently change your layout, and you only notice when something looks wrong. This checker reads your markup the way a strict reviewer would and points to the exact spot.
Errors are things that break the document: tags that are never closed or closed in the wrong order, closing tags without an opening tag, attribute quotes that never end, duplicate IDs, and images without alternative text. Warnings are likely problems such as form fields without labels, links without an address, skipped heading levels and obsolete tags like <center>. Tips are good-practice suggestions, for example adding rel="noopener" to links that open a new tab.
It understands HTML5 rules, so elements whose end tags are optional (like <li> and <p>) and void elements (like <img> and <br>) are not reported by mistake. When you paste a whole page it also checks the doctype, character set, viewport, title and language. Everything runs in your browser.
Questions and answers
Is this the same as the W3C validator?
It is a quick, friendly checker for the mistakes people make most often, with explanations aimed at learners. For formal standards compliance you can still run the official W3C Nu validator, but this tool works offline and never uploads your code.
Why is a missing </li> not reported?
In HTML5 the end tags of some elements, including li, p, td, tr and option, are optional. The browser closes them automatically, so leaving them out is valid.
How is the score calculated?
It starts at 100. Each error costs 8 points, each warning 3 and each tip 1. It is a rough guide to help you track progress, not an official grade.
Does it check CSS and JavaScript?
No. It focuses on the HTML structure, attributes and accessibility. Content inside <style> and <script> tags is skipped.
Why does every image need alt text?
Screen readers read the alt text aloud to blind and low-vision users, and it appears if the image fails to load. Decorative images should use an empty alt="" so they are skipped.