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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

The HTML Validator tool checks your HTML markup for errors and warnings in real time. It detects common issues like unclosed tags, duplicate IDs, missing required attributes, deprecated elements, and improper nesting — helping you write cleaner, standards-compliant HTML code.

使い方

Paste your HTML code into the input area and the validator will instantly analyze it. Results appear automatically showing any errors (in red) and warnings (in yellow), each with the line number, column position, and a clear description of the issue. Click “Try an example” to see the validator in action with sample HTML containing common mistakes.

特徴

リアルタイム検証 – Results update automatically as you type or paste, with no button clicks needed
エラー検出 – Finds unclosed tags, duplicate IDs, missing required attributes (alt, src, href), and mismatched nesting
Warning Detection – Flags deprecated HTML elements, block-inside-inline nesting, self-closing non-void elements, and missing doctype/structure
Precise Location – Each issue includes line and column numbers so you can quickly find and fix problems
クライアント側処理 – All validation happens in your browser with no data sent to any server

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 よくある質問

What is the difference between an HTML error and a warning?

An error indicates a definite problem that violates the HTML specification, such as an unclosed tag or a duplicate ID attribute. A warning highlights a best-practice issue or potential problem, such as using a deprecated element or nesting a block element inside an inline element. Errors should always be fixed, while warnings are strong recommendations.
Why do void elements like img and br not need closing tags?

Void elements are defined in the HTML specification as elements that cannot have any child content. Since they are always empty, the spec does not require or allow a closing tag for them. Examples include img, br, hr, input, meta, and link.
Why is having duplicate ID attributes a problem in HTML?

The HTML specification requires that every id attribute value be unique within a document. When duplicate IDs exist, JavaScript methods like getElementById return unpredictable results, CSS selectors may style the wrong element, and accessibility tools such as screen readers can become confused when navigating the page.