CSS selector priority and cascade rules determine the final application effect when multiple styles conflict. 1. The priority is determined by the selector type calculation score, inline style > ID selector > Class/Property/Pseudo-Class Selector > Element/Pseudo-Element Selector; 2. Cascading involves the priority of the style source, including user agent style, user style, developer style, !important declaration and inline style; 3. The last loaded style under the same priority covers the front; 4. Use the browser developer tools to troubleshoot style conflicts, check the override, !important usage, spelling errors and the order of introduction. Mastering these rules can help write stable and clear CSS code.

The priority and cascade rules of CSS selectors are often encountered but easily confused parts in front-end development. Understanding these rules can help you avoid many style conflicts, especially when the project is complex or when multiple people collaborate.

What is selector priority?

Simply put, priority determines which one will be applied eventually when multiple CSS rules act on the same element . Priority is not simply "over the front", but rather calculates the score based on the type of selector, and the rule with high scores wins.

For example, the following example:

 p {
  color: red;
}

#main p {
  color: blue;
}

Even if the <p></p> tag is written in front of the CSS file, the color will turn blue as long as it is selected by the higher priority selector #main p .

The priority is calculated like this (from high to low):

• The inline style has the highest priority (for example: style="color: red" )
• ID selector (such as: #header )
• Class selector, attribute selector, pseudo-class (such as: .btn , [type="text"] , :hover )
• Element selector, pseudo-element (such as: div , ::before )

You can remember in a four-digit way:
內(nèi)聯(lián)(1,0,0,0) > ID(0,1,0,0) > 類等(0,0,1,0) > 元素等(0,0,0,1)

What's the matter with the stacking order?

In addition to priority, there is also a concept called Cascading that will also affect the final style.

Cascading refers to how the browser decides who takes effect from multiple sources, including:

• User Agent Style Sheet (Browser Default Style)
• User-defined styles (such as user-set font size)
• Styles written by developers (your CSS file)
• Important statement (using !important style)
• Inline style

There is also a priority relationship between them, in the rough order:

• Normal style (normal writing)
• !important style (use with caution)
• The last loaded style of the same priority will overwrite the previous one

So if you write two identical class names and have the same priority, then the subsequent rule will override the previous one .

How to tell which rule works?

In actual development, you can use the browser's developer tools (F12 or right-click check) to see which styles are applied to an element and where these styles come from.

Here are a few tips to help you make a quicker judgment:

• Look at the crossed style in DevTools, which means it is overwritten
• Pay attention to whether it is used !important
• Check for spelling errors or mismatched selectors
• View the order of introduction, especially if multiple CSS files are

For example: If you have a .button class but you see the style of another .btn class on the page, it may be because the two have the same priority and .btn is written later in the file.

Summarize the key points

In general, understanding CSS priority and cascade mechanisms can help you write clearer and more stable style code. Remember a few key points:

• Priority is determined by selector composition, ID > Class > Element
• !important is strong, but don't abuse it
• Under the same priority, the following style will overwrite the previous one.
• Use developer tools to quickly troubleshoot problems

Basically, that's all. Although it looks a bit abstract, you can slowly grasp the rules when paying more attention to these details during the debugging process.

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