<bdi> 解決了在混合文本方向環(huán)境中動(dòng)態(tài)內(nèi)容的正確顯示問題，它通過自動(dòng)檢測並隔離未知方向的文本，確保標(biāo)點(diǎn)符號(hào)和中性字符正確渲染。 1. 當(dāng)在已知方向的文檔中嵌入未知方向的文本（如阿拉伯語用戶名在英文頁面）時(shí)，瀏覽器的Unicode雙向算法可能錯(cuò)誤排列文本；2. <bdi> 元素使瀏覽器能自動(dòng)判斷內(nèi)部文本方向並將其與周圍內(nèi)容隔離；3. 適用於用戶生成內(nèi)容、動(dòng)態(tài)數(shù)據(jù)和多語言界面；4. 與需預(yù)知方向的 和dir 屬性不同，<bdi> 具備智能檢測能力；5. 現(xiàn)代瀏覽器均支持<bdi>，IE 不支持但可通過降級(jí)方案處理。因此，在顯示非自己編寫的多語言內(nèi)容時(shí)，應(yīng)使用<bdi> 標(biāo)籤以避免佈局錯(cuò)亂。

The HTML <bdi></bdi> element is specifically designed to handle bidirectional text — content that may be displayed in either left-to-right (like English) or right-to-left (like Arabic or Hebrew) scripts — without requiring manual intervention to ensure correct text rendering.

This is especially useful when you're displaying user-generated or dynamic content whose language and direction you can't predict in advance.

What problem does <bdi></bdi> solve?

In bidirectional text, browsers use the Unicode Bidirectional Algorithm (UBA) to determine how text should be displayed. However, when embedding text of an unknown direction within a document of a known direction (eg, inserting Arabic names into an English page), the algorithm can get confused — especially with punctuation, numbers, or mixed-language strings — leading to incorrectly ordered text.

For example, imagine an English (LTR) page displaying a list of usernames, some of which are in Arabic (RTL). Without proper isolation, the browser might incorrectly position parentheses or numbers next to the username.

 <!-- Without bdi — might render incorrectly -->
<p>User: ???? (Online)</p>

The parentheses or other neutral characters might end up in the wrong place.

How <bdi> helps

The <bdi> element tells the browser: "Treat this piece of text as directionally isolated from its surroundings." It automatically detects the correct direction based on the content.

 <p>User: <bdi>????</bdi> (Online)</p>

Now the browser will:

• Determine the direction of the text inside <bdi> (in this case, RTL due to Arabic).
• Isolate that text from surrounding LTR context.
• Render the parentheses and other neutral characters correctly.

When to use <bdi>

Use <bdi> in these common scenarios:

• User-generated content : Displaying usernames, comments, or posts where language and direction are unknown.
• Dynamic data : Rendering names, titles, or labels from a database or API that may contain mixed scripts.
• Multilingual interfaces : Applications supporting both RTL and LTR languages.

Examples:

 <ul>
  <li>User: <bdi>JohnDoe123</bdi> (Online)</li>
  <li>User: <bdi>????_2023</bdi> (Online)</li>
  <li>User: <bdi>張偉</bdi> (Online)</li>
</ul>

Each of these will be directionally isolated, so punctuation and layout stay correct regardless of script.

Key differences from <bdo></bdo> and dir

• <bdo></bdo> (bidirectional override) forces a specific direction, ignoring natural text direction. Use it only when you need to override the default behavior.
• The dir attribute sets direction explicitly ( dir="rtl" or dir="ltr" ), but requires you to know the direction in advance.
• <bdi></bdi> is "smart" — it auto-detects direction and isolates the text, making it ideal for unpredictable content.

Browser support and fallback

<bdi></bdi> is supported in all modern browsers, including recent versions of Chrome, Firefox, Safari, and Edge. Internet Explorer does not support it, so if you need to support older environments, consider:

• Polyfilling (though limited),
• Using dir attributes with language detection,
• Or gracefully accepting minor rendering issues in rare cases.

Summary

• Use <bdi></bdi> for text with unknown or variable direction.
• It automatically isolates and detects text direction.
• Ideal for usernames, dynamic content, and multilingual apps.
• Prefer over <bdo></bdo> or manual dir when direction isn't known upfront.

Basically, if you're displaying text you didn't write yourself and it might be in any language, wrap it in <bdi></bdi> . It's a small tag that prevents big layout surprises.

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