什麼是用戶代理樣式表？

用戶代理樣式表是瀏覽器自動應用的默認CSS樣式，用於確保未添加自定義樣式的HTML元素仍具基本可讀性。它們影響頁面初始外觀，但不同瀏覽器存在差異，可能導致不一致顯示。開發者常通過重置或標準化樣式來解決這一問題。使用開發者工具的“計算”或“樣式”面板可查看默認樣式。常見覆蓋操作包括清除內外邊距、修改鏈接下劃線、調整標題大小及統一按鈕樣式。理解用戶代理樣式有助於提升跨瀏覽器一致性並實現精準佈局控制。

User agent stylesheets are the default CSS styles that browsers apply to web pages automatically. Every browser has its own set of default styles, which means if you don't write any CSS yourself, elements like headings, paragraphs, lists, and links will still have some basic styling — just not a consistent one across browsers.

Why user agent styles matter

You might not think about them often, but these default styles affect how your page looks before your own CSS kicks in. For example, most browsers make links blue and underlined, headings bold and larger, and lists indented. These choices aren't random — they're part of the browser's attempt to make HTML readable even with no custom styling.

The problem is, different browsers sometimes style things slightly differently by default. That's why developers often start projects by resetting or normalizing styles — so everything starts from a more predictable baseline.

How to see user agent styles in action

If you're curious what styles your browser applies by default, you can check using developer tools:

• Open DevTools (right-click on a page and choose "Inspect")
• Select an element like a <button></button> or <h1></h1>
• Look at the "Computed" tab or the "Styles" pane
• You'll see entries labeled (user agent stylesheet) next to default styles

This can be super helpful when trying to figure out why something looks the way it does without your own CSS touching it.

When you should override them

Most of the time, you'll want to override user agent styles to keep your site looking consistent across browsers. A few common examples:

• Removing default margins and padding with a reset like * { margin: 0; padding: 0; }
• Changing link underline behavior
• Adjusting heading sizes to match your design system
• Making buttons look the same in Chrome, Firefox, and Safari

Even modern CSS resets or libraries like Normalize.css exist largely to smooth over differences between user agent stylesheets.

So yeah, user agent stylesheets are just the built-in rules browsers use to display HTML. They help make unstyled pages readable, but they can also cause inconsistencies — especially if you're trying to build a pixel-perfect layout. Knowing how they work helps you take control and make sure your site looks the way you intend.

基本上就這些。

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