HTML Validator
One of the most important steps after creating a new web page is validating your HTML code. Validation helps ensure that your markup follows established web standards, making your page more reliable, accessible, and compatible across different browsers and devices.
When HTML is written correctly, browsers can interpret and display your content consistently. Invalid or poorly structured code may still appear to work in some browsers, but it can cause unexpected layout issues, broken functionality, or accessibility problems in others. Validation helps identify errors such as missing tags, incorrect nesting, deprecated elements, and invalid attributes-issues that can be difficult to spot manually.
Another major benefit of validation is future-proofing your website. Web standards evolve over time, and pages built with clean, valid HTML are far more likely to remain compatible with future browsers, updates, and technologies. This reduces maintenance issues and helps ensure long-term stability.
HTML Validator
Results
Note: This validator will only validate HTML code, if you need to validate CSS code, please use this CSS Validator.
About HTML Validator
Online HTML validator is a webool that checks the HTML (Hypertext Markup Language) code of a web page to ensure that it conforms to the official HTML specifications and standards defined by the World Wide Web Consortium (W3C). The W3C sets the standards for HTML, and these standards dictate how web pages should be structured and formatted to ensure consistency and compatibility across different web browsers and devices.
HTML validators are used to identify and report any errors or issues in the HTML code of a web page. These errors could include:
- Missing or improperly nested HTML tags.
- Incorrect attributes or values within HTML tags.
- Deprecated or obsolete HTML elements.
- Structural issues that could impact the accessibility or usability of a web page.
- Incomplete or improperly closed tags.
- Incorrect usage of character encoding.
By running HTML code through our HTML validator, web developers and designers can ensure that their code follows best practices and is more likely to display correctly and consistently across different browsers and devices. It can help improve the quality, accessibility, and search engine optimization (SEO) of a website.
What does it check?
The validator runs over 60 distinct checks across seven categories:
| Category | What is checked |
|---|---|
| Structure | DOCTYPE, html/head/body presence, tag nesting, unclosed tags, duplicate IDs, void element misuse, misnested elements |
| HTML5 | Deprecated elements and attributes, self-closing syntax, uppercase tag names, presentational attributes, redundant type attributes |
| Accessibility | Missing alt text, label/input associations, empty links and buttons, heading hierarchy, ARIA roles, tabindex values, video captions |
| Forms | Missing method attribute, nested forms, unlabelled inputs, select without options, textarea without name |
| Attributes | Unquoted attribute values, invalid input types, invalid target values, boolean attributes with false values, deprecated presentational attributes |
| Document | Meta charset placement and duplication, viewport meta, title element, duplicate singleton elements, base element placement |
| Semantics | Multiple main elements, empty headings, empty paragraphs, internal anchor links, label-for/id pairing, obsolete name attribute on anchors |
How is this different from the W3C validator?
The W3C Nu HTML Checker is the official reference validator and is excellent for a full specification-level check. The CSSPortal HTML Validator takes a different approach - it is purpose-built for practical, everyday use:
- Instant results with no network round-trip to an external service
- Privacy-first - your HTML never leaves CSSPortal's infrastructure
- HTML5 semantic elements are understood natively, not flagged as unknown
- Results are presented in plain language with line numbers and rule identifiers
- Fragments and full documents are both supported and auto-detected
Who is it for?
The HTML Validator is useful for anyone who writes or maintains HTML markup:
- Front-end developers who want to catch errors before committing or deploying
- Web designers checking that their templates and components are well-formed
- Content editors pasting HTML into CMS platforms who want to verify their markup
- Students learning HTML who want immediate, clear feedback on their code
Frequently Asked Questions
Is the HTML Validator free to use?
Yes, completely free. There are no usage limits, no account required, and no hidden costs. The tool is provided as a free resource by CSSPortal.
Is my HTML code kept private?
Yes. Your code is processed on CSSPortal's own servers and is never forwarded to any third-party service or API. Nothing is stored, logged, or shared. You can safely validate sensitive or proprietary markup.
What is the difference between an error, a warning, and an info message?
Errors indicate invalid markup that browsers may handle inconsistently or that breaks the HTML specification - these should always be fixed. Warnings flag code that is technically valid but problematic in practice, such as deprecated attributes or missing accessibility features. Info messages are advisory notes about best practices, such as a missing method attribute on a form.
Can I validate HTML fragments as well as full documents?
Yes. The validator automatically detects whether you have pasted a full HTML document (with a DOCTYPE and html/head/body structure) or a fragment (such as a component or snippet). Checks that only apply to full documents - like DOCTYPE presence and meta charset - are skipped automatically for fragments.
Why does the validator not flag my HTML5 semantic elements as errors?
This validator is built specifically for HTML5 and treats all standard HTML5 elements - including section, article, main, header, footer, nav, figure, details, and others - as valid. Some older validators are built on legacy parsers that do not recognise these elements; this tool does not have that limitation.
Does it check accessibility?
Yes. The validator checks for a range of common accessibility issues including missing alt attributes on images, inputs without associated labels, empty links and buttons, skipped heading levels, invalid ARIA roles, positive tabindex values, and video elements without caption tracks. These checks are based on WCAG guidelines.
Why is an unclosed tag only a warning and not an error?
Some tags - such as li, p, dt, dd, td, and option - are permitted by the HTML5 specification to have their closing tags omitted in certain contexts. Browsers handle these cases consistently through implicit closing rules. The validator flags them as warnings rather than errors to reflect that the markup will work but is not considered best practice.
Can I use this to validate HTML email templates?
You can paste HTML email code and the structural checks will run as normal. However, email clients do not follow the HTML5 specification and have their own rendering quirks - some deprecated elements and attributes flagged by this validator are still commonly used in email markup for compatibility reasons. Use the results as a guide rather than a strict pass/fail.
What does the line number in a result refer to?
Each result includes the line number in your original HTML where the issue was detected. For issues such as unclosed tags, the line number points to where the opening tag appeared - not where the end of the document was reached - so you can find the problem quickly.
Does the validator check inline CSS or JavaScript?
The validator checks for the presence and placement of script and style elements - for example, flagging render-blocking scripts in the head without async or defer, and redundant type attributes. It does not parse or validate the contents of CSS or JavaScript code within those elements. For CSS validation, CSSPortal also offers a dedicated CSS Validator.