CSS style cascading means that when multiple CSS rules are applied to the same element, the final applied style is determined based on the priority of the rule and the specificity of the rule.
In web development, the cascading nature of styles is very important. Through cascading, developers can easily provide flexibility in website design and layout, and make styles more consistent and easier to maintain. Understanding the principles and usage of style cascading is essential basic knowledge for every front-end developer.
First of all, CSS style cascading determines which style will be applied to the element based on the priority of the rule. The priority is divided into four levels from high to low:
- Inline Style: The style specified directly in the style attribute of the HTML element tag has the highest priority. For example:
<div style="color: red;">Hello World!</div> - ID Selector: Use
The # symbol plus a unique ID specifies the style. For example:#myId { color: blue; } - Class and Attribute Selectors: Use the
.symbol plus the class name or Use the[]symbol plus the attribute name to specify the style. For example:.myClass { color: green; }or[type="text"] { border: 1px solid black; } - Tag selector and pseudo-class Selectors (Tag and Pseudo-class Selectors): Selectors that specify element tag names or specific states, such as
a,div:hover. For example:h1 { font-size: 20px; }ora:hover { text-decoration: underline; }
Secondly, at the same priority Among the rules, specificity affects style cascading. Specificity is a value used to measure the relative weight of style rules. It consists of four parts: the weight of inline styles, the weight of ID selectors, the weights of class selectors and attribute selectors, label selectors and The weight of the pseudo-class selector. Among them, inline styles have the highest weight, followed by ID selectors, class selectors and attribute selectors, and label selectors and pseudo-class selectors have the lowest weights. Rules with higher specificity values have higher priority and will override rules with lower specificity values.
In addition to the above two points, there are some other rules and special cases that will affect the cascading nature of CSS styles:
- !important rules: Using !important rules in styles can The priority of this style rule is raised to the highest level. Use the !important rule with caution, as overuse can make CSS code difficult to maintain.
- Inheritance: Some style attributes are inheritable, that is, child elements will inherit the style of the parent element. When a child element and a parent element have styles with the same attribute, the child element's style will override the parent element's style.
The following is a specific CSS code example to illustrate the use of style cascading:
<!DOCTYPE html>
<html>
<head>
<style>
/* 内联样式 */
p {
color: red !important;
}
/* ID选择器 */
#myId {
color: blue;
}
/* 类选择器和属性选择器 */
.myClass {
color: green;
}
/* 标签选择器和伪类选择器 */
a {
color: purple;
}
</style>
</head>
<body>
<div id="myId" class="myClass">
<p>Hello World!</p>
<a href="#">Visit us</a>
</div>
</body>
</html>In the above example, first we give p The element has an inline style added with the highest priority, sets its color to red, and uses the !important rule. Next, we set an ID selector style for the div element and set its color to blue. Then, we added a class selector style and a label selector style to the div element, with colors green and purple respectively.
Eventually, the color of the p element will have the red color of the inline style applied, while the color of the div element will have the ID selector's blue style applied. Because of specificity rules, class selector styles and label selector styles are ignored. So, the final output is "Hello World!" in red and "Visit us" in blue.
In summary, CSS style cascading determines the final applied style through the priority and specificity of the rules. Understanding the principles of cascading and learning to flexibly use the rules of cascading will help developers better control and manage CSS styles and realize various design needs of the website.
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