id和class在CSS中的作用不同，使用場景也不同。 1. id是唯一的標識符，用於單個獨特元素的樣式設計，如#main-content；2. class可以重複使用，適用於多個元素應用相同樣式，如.button；3. id的選擇器優(yōu)先級高於class，會影響樣式覆蓋；4. HTML中id用id="name"，class用class="name"；5. CSS中id用#name選擇，class用.name選擇；6. 推薦避免過度使用id進行樣式設計，因不易覆蓋；7. 同一元素可同時使用id和class；8. 若需通過JavaScript操作元素，使用id更方便。

When you're styling HTML elements with CSS, two of the most commonly used selectors are id and class . While they both target elements on the page, they serve different purposes and have distinct behaviors.

One Unique ID, Many Reusable Classes

An id is meant to be unique within a page — you should only use one id per element, and that same id shouldn't appear on more than one element. This makes id perfect for styling elements that only appear once, like a header or a main navigation bar.

A class , on the other hand, can be used on multiple elements. That makes it ideal when you want to apply the same style to several different elements. For example, if you have multiple buttons that should look the same, giving them the same class makes your CSS cleaner and more maintainable.

Here's a quick comparison:

• Use id for one-of-a-kind elements.
• Use class for repeated styling across multiple elements.

Specificity Matters

One key difference that often trips people up is specificity . In CSS, when two rules apply to the same element, the one with higher specificity wins.

An id selector has higher specificity than a class selector. That means if you have conflicting styles — say, a class sets the text color to blue and an id sets it to red — the id rule will take precedence.

This can be helpful when you want to override general styles for a specific element, but it can also lead to hard-to-debug issues if you're not careful.

HTML and CSS Syntax Differences

The syntax for using id and class is slightly different in both HTML and CSS.

In HTML:

• id is written as id="header"
• class is written as class="button"

In CSS:

• id is selected with a hash: #header { ... }
• class is selected with a dot: .button { ... }

It's a small difference, but it's important to get right. Using the wrong symbol in your CSS will mean your styles don't apply at all.

When to Use Which?

Here are a few practical guidelines:

• Use id when targeting a single, unique element (like #main-content or #sidebar )
• Use class when applying styles to multiple elements (like .highlight or .nav-link )
• Avoid overusing id for styling — it can make your CSS harder to override later
• You can use both id and class on the same element if needed

Also, JavaScript often uses id to target specific elements, so if you plan to manipulate an element with JS, using an id might make things easier.

基本上就這些。

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