How does CSS priority work?

Question

Let's say I have the following HTML:

<span id="id1" class="class1 class2">This is a test</span>

and if #id1, .class1 and .class2 all have different sets of mutually exclusive CSS rules, which one wins? I have been testing an example and in one case it's choosing (I think) the one that is listed at the bottom of the CSS file, but in another case it seems non-deterministic.

Is there a specific rule in this case?

https://developer.mozilla.org/en-US/docs/CSS/Specificity – Matt Ball Apr 13 '13 at 02:51 — Matt Ball, Apr 13 '13 at 02:51

score 7 · Accepted Answer · edited May 23 '17 at 12:12

The basic principle of cascading in CSS is that you have one element, and one or more CSS rules that apply to the same element (because the element matches their selectors). In this process, all applicable styles are computed, with any conflicts resolved (or cascaded), and then, well, applied.

If the rules and their declarations are mutually exclusive, then none of them "wins" over any of the others per se, since there's no conflict to resolve and therefore nothing to override. For example, if you have these rules:

#id1 {
    color: red;
}

.class1 {
    border-width: 1px;
}

.class2 {
    border-style: dashed;
}

Then your element will have red text and a dashed red border that's 1 pixel thick. There are no conflicts, so all of them will combine in effect. (Note that the border is red due to special behavior.)

jsFiddle preview

It's only when you have the same property declared in more than one rule that selector specificity and cascading become relevant, because then you'd need to override values for that same property. In that case, then as already mentioned IDs take precedence over classes and equally-specific rules are applied from the top down; read about selector specificity.

For example, if you have these rules:

#id1 {
    color: red;
}

.class1 {
    text-decoration: underline;
    color: green;
}

.class2 {
    text-decoration: none;
    color: blue;
}

Then your element will have red text with no decoration, because

the color value in #id1 overrides that in both classes, and
the text-decoration value in .class2 overrides that in .class1.

jsFiddle preview

Remember that all this has to apply to the same element. If you have a parent that only has an ID, with a child that only has a class, then none of this will apply because you're dealing with entirely separate elements. What comes into play instead is inheritance, which is also covered in the document I link to above.

score 0 · Answer 2 · answered Apr 13 '13 at 02:51

0

CSS reads up to down, so class2 will win!

More Info http://css-tricks.com/specifics-on-css-specificity/

answered Apr 13 '13 at 02:51

Axel A. García

683
9
21

so Id doesn't trump class? – leora Apr 13 '13 at 02:52
Not necessarily it depends upon where you have defined class2. if it is above class1 in the css, then it will lose to class1. – PSL Apr 13 '13 at 02:55
Well ID wins. Though you were asking about classes... :P It goes like this: **STYLE ATTRIBUTE > ID > CLASS > TAG(IN CSS).** – Axel A. García Apr 13 '13 at 02:55

Eli · Answer 3 · 2013-04-13T03:03:31.160

ID will take precedence over class.

If an element has same styles defined multiple times using ID, the latter will take precedence over the former except the case that you're using !important

The order of precedence with CSS is as follows:

1. !important

2. Inline styles which defined inside an HTML element

3. Internal styles which defined in the head section

4. External stylesheet which is a link to a sheet (<link rel="stylesheet" type="text/css" href="style.css" />)

5. Browser default

score 0 · Answer 4 · answered Apr 13 '13 at 04:32

This is easy - the one that is closer and more refined win (just like life)

I.e.

Got a blank sheet then:

Start off with class - apply those (aka class1, class2)

But you know about that individual (id)

And then apply that..

So (if there is a "winner") the id wins

Unless the tag has style in its attributes - That would win

How does CSS priority work?

4 Answers4