ID選擇器以#開頭，用於唯一元素；類選擇器以.開頭，可複用於多個元素。例如：#header定義唯一標題樣式，.button定義多元素按鈕樣式。優(yōu)先用ID定位唯一組件，用類統(tǒng)一多元素風格。組合選擇如div.content或#sidebar .button可精準控製樣式。掌握二者區(qū)別與組合應(yīng)用能提升CSS效率與維護性。

To target HTML elements by ID and class in CSS, you use specific selector syntax. IDs are unique identifiers meant for a single element on the page, while classes can be reused across multiple elements. Here's how you work with both.

Using ID Selectors

An ID selector starts with a # followed directly by the ID name. Since IDs are unique, this selector targets exactly one element on the page.

For example:

 #header {
  background-color: #333;
  color: white;
}

This rule will apply only to the element with id="header" .

You'll typically use ID selectors when styling major layout components like headers, footers, or unique containers. Just keep in mind that because they're unique, you can't reuse them without breaking HTML rules.

Styling with Class Selectors

Class selectors start with a dot ( . ), followed by the class name. They're ideal for applying consistent styles to multiple elements.

Example:

 .button {
  padding: 10px 20px;
  border-radius: 5px;
  font-weight: bold;
}

Any element with class="button" will get these styles — perfect for things like buttons, badges, or repeated UI elements.

You can also combine multiple classes on a single element:

 <div class="button primary">Click Me</div>

Then style each separately:

 .primary {
  background-color: blue;
  color: white;
}

When to Use ID vs Class

Choosing between an ID and a class mostly comes down to uniqueness and specificity :

• Use an ID if the element is one-of-a-kind (like a main navigation bar or hero section).
• Use a class when applying the same look to multiple items (like product cards or alert messages).

Also note that ID selectors have higher specificity than classes. That means styles applied via ID are harder to override — something to keep in mind when debugging layout issues.

Combining Selectors for More Precision

Sometimes you need more control over which elements get styled. You can combine selectors to narrow things down.

Here are a few useful combinations:

• Target a specific element with a class:

   div.content { /* Styles only <div class="content"> */ }

• Style an element inside another:

   #sidebar .button { /* Targets .button only inside #sidebar */ }

• Chain multiple classes together:

   .btn.large.primary { /* Applies only if all three classes are present */ }

  These techniques help you write more targeted, maintainable styles without relying on overly generic rules.

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  That's the core of using ID and class selectors in CSS — straightforward but powerful. The trick is knowing when to use each and how combining them can help you avoid unnecessary overrides or messy code.

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