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HTML Validator

開発者文章

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入力

自動処理

出力

クライアント側

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ガイド

HTML Validator

Validate your HTML markup instantly for syntax errors and W3C compliance. This free online HTML validator checks for unclosed tags, missing required attributes, deprecated elements, duplicate IDs, invalid nesting, and structural issues — all in real time as you type.

使い方

1. Paste your HTML code into the input textarea, or click “Try an example” to load sample HTML with common errors.
2. Validation results appear automatically as you type — no button click needed.
3. Review each issue with its line number, column, and severity (error or warning).
4. Fix your HTML based on the feedback and re-validate until all issues are resolved.

特徴

Real-time Validation – Results update instantly as you type with no page reload
Unclosed & Mismatched Tags – Detects missing closing tags and tag nesting errors
Required Attributes – Checks for missing alt on images, href on links, and more
Deprecated Elements – Flags obsolete HTML tags like center, font, and marquee
Duplicate ID Detection – Finds elements sharing the same ID attribute
Invalid Nesting – Warns when block elements are placed inside inline elements
Document Structure – Checks for DOCTYPE, html, head, and body elements
Line & Column Numbers – Pinpoints exact location of each issue in your code

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 よくある質問

What is HTML validation and why does it matter?

HTML validation is the process of checking HTML markup against the rules defined by the W3C specification. Valid HTML ensures consistent rendering across browsers, improves accessibility for screen readers, helps search engines parse your content correctly, and reduces unexpected layout bugs.
What is the difference between HTML errors and warnings?

Errors indicate violations that will likely cause rendering problems or broken functionality, such as unclosed tags or missing required attributes. Warnings highlight best practice issues that may not break the page but could affect accessibility, SEO, or future compatibility, such as deprecated elements or missing document structure.
What are void elements in HTML?

Void elements are HTML tags that cannot have any content and do not need a closing tag. Common examples include img, br, hr, input, meta, and link. Using a closing tag on a void element is unnecessary and considered incorrect in HTML5.
Why should block elements not be nested inside inline elements?

The HTML specification defines a content model for each element. Inline elements like span, a, and em are designed to contain text and other inline content. Placing block-level elements like div or p inside them violates the specification and can cause unpredictable rendering behavior across different browsers.