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What Is HTML Validation?
HTML validation checks whether your markup conforms to the HTML specification. Valid HTML ensures that browsers render your page consistently, assistive technologies can interpret your content correctly, and search engines can crawl your site without issues.
While modern browsers are forgiving with malformed HTML, invalid markup can cause subtle rendering bugs, accessibility problems, and SEO penalties. A validator catches unclosed tags, missing attributes, deprecated elements, and structural issues that are easy to miss during development.
How the HTML Validator Works
CheckTown's HTML validator parses your markup and reports errors with line numbers and clear descriptions.
- Tag validation — checks for unclosed tags, improperly nested elements, and unknown tag names
- Attribute checking — verifies required attributes are present and attribute values follow the specification
- Structure analysis — ensures proper document structure including doctype, head, and body organization
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Validate Your HTML →Common HTML Errors and How To Fix Them
Certain HTML mistakes appear repeatedly across websites. Knowing the most common issues helps you write cleaner markup from the start.
- Unclosed tags — forgetting to close a div, span, or paragraph tag can break the entire page layout downstream
- Missing alt attributes — images without alt text fail accessibility requirements and hurt SEO performance
- Incorrect nesting — placing block elements inside inline elements violates the HTML specification and causes unpredictable rendering
When To Use HTML Validation
HTML validation fits into multiple stages of the development workflow, from writing code to deploying to production.
- During development — validate early and often to catch structural errors before they compound into layout bugs
- Code reviews — run validation as part of your review process to maintain consistent markup quality across the team
- SEO audits — validate HTML as part of technical SEO checks to ensure search engines can parse your pages correctly
Frequently Asked Questions
Does invalid HTML affect SEO?
Yes. While search engines can handle minor HTML errors, significant structural issues like unclosed tags, missing heading hierarchy, or broken semantic markup can prevent crawlers from understanding your content properly, potentially affecting rankings.
What is the difference between HTML validation and linting?
HTML validation checks whether markup follows the official HTML specification. Linting goes further by enforcing code style rules, best practices, and project-specific conventions. Both are valuable but serve different purposes.
Should I fix all HTML validation warnings?
Errors should always be fixed as they indicate specification violations. Warnings are recommendations — prioritize those related to accessibility and semantic structure, as they have the most practical impact on usability and SEO.