To solve the problem that CSS styles are not effective or are overwritten, you need to understand and deal with specificity. First, CSS priority is determined by selector type: Inline Style > ID Selector > Class/Attribute Selector and Pseudo Class > Tag Selector and Pseudo Elements. Secondly, use browser developer tools to check the style and coverage of the application. Third, pay attention to the differences in writing methods of repeat selectors and the impact of third-party libraries. Fourth, avoid abuse!important, and give priority to improving selector specificity, such as adding parent class name. Finally, rationally organize the CSS structure, keep it simple and modular, use BEM nomenclature, and reduce ID use, thereby reducing conflicts and improving maintenance efficiency.

Sometimes, even though I clearly wrote the CSS style, I found that the page was not effective at all, or it was overwritten by other styles. This situation is likely to be a problem with CSS specificity. Don’t worry, this is not a rare disease, it is a common "headache" in front-end development. By figuring out a few key points, you can quickly locate and solve them.

Understand the basic rules of CSS specificity

The browser rendering mechanism of CSS determines that when multiple rules act on the same element, the one with higher priority will win. And this priority is determined by the specific value of the selector.

Simply put:

• The inline style has the highest specificity (such as style="color: red" )
• ID selector followed (such as #header )
• Classes, attribute selectors and pseudo-classes are followed (such as .btn , [type="text"] , :hover )
• Tag selector and pseudo-element lowest (such as div , ::before )

For example:

 /* Specificity: 0,1,0 */
div#main {
  color: red;
}

/* Specificity: 0,0,2 */
.content:hover {
  color: blue;
}

Although the second one is written at the back, the first one has a higher priority, so the text color is still red.

Use developer tools to view the style of the actual application

When the style does not take effect, the first reaction should be to open the browser's developer tool (F12 or right-click to check), find the corresponding element, and take a look:

• Which styles are really applied?
• Which styles are crossed out (that is, they are overwritten)?

Under the "Computed" tab, you can see the final calculated style, and you can also see which rule wins in the end. Combined with the "Styles" panel, you can clearly see which rule was killed because of insufficient priority.

Skill:

• See if there are duplicate selectors, but the writing method is different but the priority is different
• Pay attention to whether the styles that come with third-party libraries or frameworks affect your custom styles

Avoid overuse!important (but know it can help)

!important is a means of "forced override", but it destroys normal priority logic and easily makes the code more and more messy. However, in some emergency situations (such as you cannot modify the style structure of a certain component), add an !important one to save the emergency.

A better approach is:

• Improve the specificity of your own selector, rather than using it directly !important
• For example, change .btn to .container .btn , so the priority will be improved.

Of course, if you are writing component library or theme styles, you may need to use it properly !important to ensure that the core styles are not easily overwritten.

Reasonably organize CSS structures to avoid the source of conflicts

Many specific problems are actually caused by the fact that CSS is written too messy and has no unified norm. You can start from these aspects:

• Keep the selector simple : don't write it at any time.container .container > div:nth-child(2) .content
• Divide CSS files by module : such as header.css, footer.css to avoid global pollution
• Use BEM nomenclature : naming methods like .card__title--highlighted are both semantic and not easy to conflict
• Avoid abuse of ID selectors : ID priority is too high, unless necessary, try to use class instead

After reasonable organization, you will find that the situation of style conflicts is greatly reduced and it is easier to maintain.

Basically that's it. CSS specificity issues seem complex, but as long as you master the priority rules and developer tool structure specifications, most of them can be solved quickly. It's not a metaphysics, it's just that the details are easy to ignore.

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