HTML ID와 클래스의 차이점은 무엇입니까?

id必须唯一，每个页面中一个id只能用于一个元素，而class可重复使用于多个元素，且一个元素可拥有多个class；2. 使用id的场景包括：定位单个特定元素、页面内链接锚点、JavaScript通过id操作元素、label与表单元素关联；使用class的场景包括：对多个元素应用相同样式或行为、构建可复用UI组件、JavaScript选择多个元素；3. CSS中通过#id选择器和.class选择器分别 targeting，JavaScript中getElementById()用于id，getElementsByClassName()或querySelector()用于class；4. 最佳实践中应避免重复id，以免破坏可访问性和脚本功能，推荐用class进行样式定义，仅在需要唯一标识时使用id；总之，id用于唯一标识（每页仅一次），class用于可复用样式或分组（可多次使用），若需命名特定页面部分用id，若需对相似元素统一处理则用class。

The main difference between HTML id and class lies in their uniqueness and usage purpose.

1. Uniqueness: One vs. Many

• id must be unique within a page. Each element can have only one id, and no two elements should share the same id.
  Example:

  <h1 id="page-title">Welcome</h1>
  <p id="page-title">This is wrong!</p> <!-- Don't do this -->

• class can be reused across multiple elements. One element can also have multiple classes. Example:

  

  <p class="highlight">Important note</p>
  <div class="highlight warning">Another important note</div>

2. Use Cases

• Use id when:

  • You need to target a single, specific element.
  • You're linking to a section of the page (e.g., #section-about in URLs).
  • You're using JavaScript to manipulate a particular element by ID (document.getElementById()).
  • You're associating a <label> with a form input using the for attribute.

• Use class when:

  
  • You want to apply the same style or behavior to multiple elements.
  • You're building reusable UI components (like buttons, cards, alerts).
  • You're using JavaScript to select multiple elements (e.g., document.querySelectorAll('.active')).

3. CSS and JavaScript Targeting

• In CSS:

  #header { color: blue; }     /* Targets element with id="header" */
  .button { color: red; }      /* Targets all elements with class="button" */

• In JavaScript:

  document.getElementById('header');    // Fast, direct access
  document.getElementsByClassName('button'); // Returns a collection
  document.querySelector('.highlight'); // Can target by class, more flexible

4. Accessibility and Best Practices

• Avoid using the same id more than once — it can break accessibility and JavaScript behavior.
• Prefer class for styling unless you specifically need a unique identifier.
• id values are often used in page navigation (e.g., example.com/page#contact scrolls to <div id="contact">).

In short:
Use id for unique identifiers (one per page), and class for reusable styles or groups (can be used many times).

Basically, if you're styling or grouping similar elements — use class. If you're naming a specific part of the page — use id.

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