How does CSS specificity decide which styles to apply?

问题

How does CSS determine when to apply one style over another?

I have been through the W3 CSS3 selectors document a few times, and that has helped me understand how to better use CSS selectors in jQuery, but it has not really helped me understand when one CSS rule will be applied over another.

I have the following the HTML:

<div class='item'>
    <a>Link 1</a>
    <a class='special'>Link 2</a>
</div>

I have the following CSS:

.item a {
    text-decoration: none;
    color: black;
    font-weight: bold;
    font-size: 1em;
}

.special {
    text-decoration: underline;
    color: red;
    font-weight: normal;
    font-size: 2em;
}

Given the above, both Link 1 and Link 2 will be styled the same, as determined by the .item a CSS. Why does the style associated with .special not take precedence for Link 2?

Obviously, I can get around it like this:

.special {
    text-decoration: underline !important;
    color: red !important;
    font-weight: normal !important;
    font-size: 1em !important;
}

But, I feel like that is a hack that I have to sprinkle in due to my lack of understanding.

回答1:

It's based on IDs, classes and tags. IDs have the highest specificity, then classes then tags, so:

.item a     == 0 1 1      0 (id) 1 (class=item) 1 (tag=a)
.special    == 0 1 0
#foo        == 1 0 0
#foo .bar a == 1 1 1
#foo #bar   == 2 0 0

whichever has the most wins :) If it's a tie, whichever one comes last in the document wins. Note that 1 0 0 beats 0 1000 1000

If you want .special to apply, make it more specific: .item a.special

回答2:

I would suggest you get familiar with this for future reference. For this particular case, note point 3 under Cascading Order:

Count the number of ID attributes in the selector.

Count the number of CLASS attributes in the selector.

Count the number of HTML tag names in the selector.

If these are applied to your code, .item a has 1 class attribute + 1 HTML tag name, while .special has only one class attribute. Hence, the former wins the right to style the special link.

回答3:

http://www.w3.org/TR/CSS21/cascade.html#specificity is the official specificity specification.

But if that's TL;DR, the (too) short version is the more words you have in your selector, the higher the specificity. And with !important even higher. That's about it.

Edit: oh, I see that your link has the same information as mine. Sorry about that.

来源：https://stackoverflow.com/questions/9133570/how-does-css-specificity-decide-which-styles-to-apply

标签

css

css-selectors

css-specificity