Why HTML Validation Matters
Browsers are forgiving — they render broken HTML as best they can, which means errors often go unnoticed. But those hidden errors can cause problems:
- SEO: Search engine crawlers follow the HTML spec. Malformed HTML can prevent proper indexing.
- Accessibility: Screen readers depend on properly structured HTML. Broken HTML breaks assistive technologies.
- Cross-browser bugs: What renders fine in Chrome may look different in Safari or Firefox with invalid HTML.
- JavaScript errors: DOM-dependent JS breaks when HTML structure is unexpected.
Common HTML Errors Found by Validators
- Unclosed tags:
<div>without a matching</div> - Missing required attributes:
<img>withoutalt,<input>withouttype - Incorrect nesting:
<p><div>...</div></p>(div inside p is invalid) - Deprecated elements: Using
<center>or<font>tags - Duplicate IDs: Two elements with the same
idattribute - Missing doctype: No
<!DOCTYPE html>declaration
How to Validate HTML Online
1Open the HTML Validator tool.
2Paste your HTML code into the input area.
3Click Validate — errors and warnings appear instantly.
4Each error includes a description and line reference to help you fix it.
5Fix errors, re-paste, and validate again until no errors remain.
💡 Validation Workflow: Run your HTML through the validator after every major change. Integrate validation into your CI/CD pipeline using the W3C Markup Validation API for automated checks on every commit.
FAQ
Do I need to validate HTML if my site looks fine?
Yes — browsers silently fix many errors, but those fixes may differ between browsers. Validation ensures cross-browser consistency and future compatibility as browsers become stricter.
Is valid HTML the same as semantic HTML?
Not exactly. Valid HTML has no structural errors. Semantic HTML means using the right elements for their meaning (using <nav> for navigation, <article> for articles). Good HTML is both valid AND semantic.