The difference between . and in css

In CSS, . and # are two very common symbols. They are both identifiers used to select elements. Among them, the dot (.) is used to specify a class selector, and the pound sign (#) is used to specify an ID selector. Although these two identifiers are very similar, they have different purposes and effects in CSS. This article will delve into their differences.

1. The syntax structure of selectors is different

In CSS selectors, both class selectors and ID selectors use "." and "#" as prefixes for identifiers. However, there are certain differences in the grammatical structure when they are used. Class selectors are defined by adding a "." (dot) before the selector, while ID selectors are defined by adding a "#" (pound sign) before the selector.

For example, to find an element with the class attribute "example" on the page, you can use the class selector, as shown below:

.example {
   color: red;
}

To find the element with the id attribute "example", You can use ID selectors, as shown below:

#example {
   color: blue;
}

2. Different weights

In CSS, each selector has a weight value, which is used to specify when multiple rules are applied to The priority of the selector for the same element. The weight values ​​of class selectors and ID selectors are different. Generally speaking, since the element corresponding to the ID selector is unique, its weight value is relatively high. On the contrary, the class selector can correspond to multiple elements, so its weight value is relatively low.

When multiple selectors are used to define styles for the same element, the weight value of the selector will take effect. If you are using a class selector instead of an ID selector, you can use a more specific selector for style definition to increase the weight value.

3. Different scopes of application

Another difference between class selectors and ID selectors is that they have different scopes of application. Class selectors can be applied to multiple elements, while ID selectors can only be applied to one element with a unique id value. This is why we usually use ID selectors to define the style of a single element, and class selectors to define the common style of a group of elements.

For example, you can use the class selector to define a CSS rule set that will apply to all paragraph elements on the page:

p {
   font-size: 16px;
   font-weight: bold;
}

Use the ID selector to define a CSS rule set that will The ruleset will be applied to the only h1 element on the page:

#page-header {
   font-size: 24px;
   color: #000;
}

Summary

In CSS, both class selectors and ID selectors are identifiers used to select elements. Although they are very similar, their usage, weight, and scope of application are different. The class selector can be used for multiple elements and its weight is relatively low, while the ID selector is suitable for a single element and its weight is relatively high. Therefore, when using it, you need to choose the appropriate selector according to your specific needs.

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