Detailed introduction to CSS style override rules

Everyone knows that the full name of CSS is "cascading style sheet", but it is estimated that many people do not know the meaning of the word "cascading". In fact, "cascading" refers to the overlay of styles. When an element is applied with multiple styles and there are style attributes with the same name, the browser must select an attribute value from them. This process is called "cascading". Style overlay (this name is more popular) follows certain rules. I had always vaguely understood this rule before, but it was not until I read "CSS: The Missing Manual" these days that it suddenly became clear. Below are some of my study notes.

First of all, it needs to be made clear that many situations will cause an element to be applied with multiple styles. The rules for style coverage also need to be determined according to different situations. The specific rules are as follows.

Rule 1: When a style conflict occurs due to inheritance, the nearest ancestor wins.

The inheritance mechanism of CSS allows an element to inherit styles from the ancestor elements that contain it. Consider the following situation:

<title>rule 1</title>
<style>
body {color:black;}   
p {color:blue;}   
</style>


    <p>welcome to <strong>gaodayue的网络日志</strong></p>

strongFrom body and The color attribute is inherited in p, but since p is closer to strong in the inheritance tree, the text in strong eventually inherits the blue color of p.

Rule 2: When the inherited style conflicts with the directly specified style, the directly specified style wins.

In the above example, if the style of the strong element is also specified, such as:

strong {color:red;}

, then according to rule 2, the text in strong will eventually be displayed is red.

Rule 3: When directly specified styles conflict, the one with the higher style weight wins.

The weight of the style depends on the style selector. The weight is defined in the following table.

CSS selector weight

Label selector 1
Class selector 10
ID selector 100
Inline style 1000
Pseudo element (:first -child, etc.) 1
Pseudo class (:link, etc.) 10

You can see that the weight of the inline style>>ID selector>>Class selector>> In addition to tag selectors, the weight of descendant selectors is the sum of the weights of each item. For example, the weight of "#nav .current a" is 100 + 10 + 1 = 111.

Rule 4: When the style weights are the same, the latter wins.

Consider the following situation

<p class="byline">Written   
 by <a class="email" href="mailto:jean@cosmofarmer.%C2%A0%C2%A0%C2%A0%0D%0A%C2%A0com">Jean   
 Graine de Pomme</a></p>
12 .byline   
 a {color:red;}p   
 .email {color:blue;}

".byline a" and "p .email" both directly specify the a element above, and the weight Both are 11, and according to rule four, the final display is blue.

Since style sheets can be external or internal, Rule 4 reminds us to pay attention to the order in which external style sheets are introduced (and the order of elements), as well as external style sheets and internal styles. The position where the table appears. Generally speaking, internal style sheets appear after the introduction of all external style sheets, usually before .

Rule 5: !important style attributes are not overridden.

!important can be seen as the "golden finger" to break the above four rules when it is absolutely necessary. If you must use a certain style attribute and prevent it from being overridden, you can add !important after the attribute value. Taking the example of rule 4 as an example, ".byline a {color:red !important;}" can Force the link to appear red. In most cases, style overrides can be controlled in other ways, and !important cannot be abused.

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