Optimizing CSS selectors using relational selectors: Tips for improving selection efficiency

Optimizing CSS selectors: How to use relational selectors to improve selection efficiency

Introduction:
In front-end development, CSS selectors are a very important concept. It is used to add styles to HTML elements and control the appearance and layout of the page. However, in large projects, optimizing the efficiency of CSS selectors is particularly important. This article will introduce how to use relational selectors to improve selection efficiency, and attach specific code examples.

1. What is a relational selector?
Relational selectors refer to CSS selectors that select through the relationship between elements. They include child selectors (">"), adjacent sibling selectors (" "), universal sibling selectors ("~"), and descendant selectors (spaces).

1. Child selector (">"):
   Separate parent elements and child elements through the ">" symbol, and only select direct child elements of the parent element.
   Code example:

   ul > li {
     color: red;
   }

   The effect of the above code is to set the text color of the direct child element li of the ul element to red.

2. Adjacent sibling selector (" "):
   Separate two adjacent sibling elements by the " " symbol, and only select the sibling element immediately preceding it.
   Code example:

   h1 + p {
     color: blue;
   }

   The effect of the above code is to set the text color of the p element immediately after the h1 element to blue.

3. Universal sibling selector ("~"):
   Separate two sibling elements by the "~" symbol, and select the previous sibling element that conforms to the selector rules of all elements.
   Code example:

   h1 ~ p {
     color: green;
   }

   The effect of the above code is to set the text color of all p elements immediately after the h1 element to green.

4. Descendant selector (space):
   Separate ancestor elements and descendant elements by spaces, and select all descendant elements that match the selector rules.
   Code example:

   div p {
     font-size: 20px;
   }

   The effect of the above code is to set the font size of all p elements inside the div element to 20 pixels.

2. How to optimize CSS selector?
After understanding the basic usage of relational selectors, we will introduce how to improve selection efficiency by optimizing selectors.

1. Limit the selector range:
   We can reduce the number of elements matched by the selector by limiting the range of the selector, thereby improving selection efficiency. For example, use a selector only within a specific parent container rather than across the entire page. This way, the browser only needs to look for matching elements in the parent container, rather than traversing the entire page.
2. Avoid using universal selectors:
   The universal selector (*) is one of the slowest selectors and will match all elements on the page. Therefore, when using selectors, try to avoid using universal selectors to improve selection efficiency.
3. Don’t over-restrict the selector:
   Although restricting the selector range can improve selection efficiency, over-restricting the selector may cause the selector to fail. When optimizing selectors, there is a trade-off between selector scope and selector flexibility.
4. Merge selectors:
   If multiple selectors have the same style, they can be merged into one selector to reduce the number of selectors and thereby improve selection efficiency.

3. Practical application case examples
The following is a specific case example to improve selection efficiency by optimizing CSS selectors.

Original CSS code:

ul li a span {
  font-size: 14px;
}

Optimized CSS code:

ul a span{
  font-size: 14px;
}

By replacing the parent element selector from li to a, we can reduce one layer of element matching , thereby improving selection efficiency.

Conclusion:
By using relational selectors and optimizing the selectors, we can improve the efficiency of CSS selectors. In actual development, choosing appropriate selectors and optimization strategies based on specific circumstances can reduce the browser's computing overhead and improve page rendering performance.

(Note: The above code examples are for demonstration purposes only and do not mean they are applicable in all situations. They should be selected and optimized according to the actual scenario)

References:

1. "CSS Selector Performance Optimization", 2020, https://zhuanlan.zhihu.com/p/68593013
2. "CSS Selector Optimization Skills", Jianshu 2016, https://www.jianshu.com/p/fb059b4dc813

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