CSS selector for first element with class

我有一堆类名为red的元素，我似乎无法使用以下CSS规则选择class="red"的第一个元素：

 .red:first-child{
      border:5px solid red;
    }

 <p class="red"></p>
 <div class="red"></div>


   

This is one of the most well-known examples of authors misunderstanding how :first-child works. Introduced in CSS2, the :first-child pseudo-class represents the very first child of its parent . That's it. There's a very common misconception that it picks up whichever child element is the first to match the conditions specified by the rest of the compound selector. Due to the way selectors work (see here for an explanation), that is simply not true.

Selectors level 3 introduces a :first-of-type pseudo-class, which represents the first element among siblings of its element type. This answer explains, with illustrations, the difference between :first-child and :first-of-type . However, as with :first-child , it does not look at any other conditions or attributes. In HTML, the element type is represented by the tag name. In the question, that type is p .

Unfortunately, there is no similar :first-of-class pseudo-class for matching the first child element of a given class. One workaround that Lea Verou and I came up with for this (albeit totally independently) is to first apply your desired styles to all your elements with that class:

/* 
 * Select all .red children of .home, including the first one,
 * and give them a border.
 */
.home > .red {
    border: 1px solid red;
}

... then "undo" the styles for elements with the class that come after the first one, using the general sibling combinator ~ in an overriding rule:

/* 
 * Select all but the first .red child of .home,
 * and remove the border from the previous rule.
 */
.home > .red ~ .red {
    border: none;
}

Now only the first element with class="red" will have a border.

Here's an illustration of how the rules are applied:

<div class="home">
  <span>blah</span>         <!-- [1] -->
  <p class="red">first</p>  <!-- [2] -->
  <p class="red">second</p> <!-- [3] -->
  <p class="red">third</p>  <!-- [3] -->
  <p class="red">fourth</p> <!-- [3] -->
</div>

As a bonus, although it was introduced in Selectors 3, the general sibling combinator is actually pretty well-supported by IE7 and newer, unlike :first-of-type and :nth-of-type() which are only supported by IE9 onward. If you need good browser support, you're in luck.

In fact, the fact that the sibling combinator is the only important component in this technique, and it has such amazing browser support, makes this technique very versatile — you can adapt it for filtering elements by other things, besides class selectors:

article > p {
    /* Apply styles to article > p:first-of-type, which may or may not be :first-child */
}

article > p ~ p {
    /* Undo the above styles for every subsequent article > p */
}

Note that in order for this to work, you will need to know in advance what the default styles will be for your other sibling elements so you can override the first rule. Additionally, since this involves overriding rules in CSS, you can't achieve the same thing with a single selector for use with the Selectors API, or Selenium's CSS locators.

It's worth mentioning that Selectors 4 introduces an extension to the :nth-child() notation (originally an entirely new pseudo-class called :nth-match() ), which will allow you to use something like :nth-child(1 of .red) in lieu of a hypothetical .red:first-of-class . Being a relatively recent proposal, there aren't enough interoperable implementations for it to be usable in production sites yet. Hopefully this will change soon. In the meantime, the workaround I've suggested should work for most cases.

Keep in mind that this answer assumes that the question is looking for every first child element that has a given class. There is neither a pseudo-class nor even a generic CSS solution for the nth match of a complex selector across the entire document — whether a solution exists depends heavily on the document structure. jQuery provides :eq() , :first , :last and more for this purpose, but note again that they function very differently from :nth-child() et al. Using the Selectors API, you can either use document.querySelector() to obtain the very first match:

var first = document.querySelector('.home > .red');

Or use document.querySelectorAll() with an indexer to pick any specific match:

var redElements = document.querySelectorAll('.home > .red');
var first = redElements[0];
var second = redElements[1];
// etc

Although the .red:nth-of-type(1) solution in the original accepted answer by Philip Daubmeier works (which was originally written by Martyn but deleted since), it does not behave the way you'd expect it to.

For example, if you only wanted to select the p in your original markup:

<p class="red"></p>
<div class="red"></div>

... then you can't use .red:first-of-type (equivalent to .red:nth-of-type(1) ), because each element is the first (and only) one of its type ( p and div respectively), so both will be matched by the selector.

When the first element of a certain class is also the first of its type, the pseudo-class will work, but this happens only by coincidence . This behavior is demonstrated in Philip's answer. The moment you stick in an element of the same type before this element, the selector will fail. Taking the updated markup:

<div class="home">
  <span>blah</span>
  <p class="red">first</p>
  <p class="red">second</p>
  <p class="red">third</p>
  <p class="red">fourth</p>
</div>

Applying a rule with .red:first-of-type will work, but once you add another p without the class:

<div class="home">
  <span>blah</span>
  <p>dummy</p>
  <p class="red">first</p>
  <p class="red">second</p>
  <p class="red">third</p>
  <p class="red">fourth</p>
</div>

... the selector will immediately fail, because the first .red element is now the second p element.

The :first-child selector is intended, like the name says, to select the first child of a parent tag. The children have to be embedded in the same parent tag. Your exact example will work (Just tried it here):

<body>
    <p class="red">first</p>
    <div class="red">second</div>
</body>

Maybe you have nested your tags in different parent tags? Are your tags of class red really the first tags under the parent?

Notice also that this doesnt only apply to the first such tag in the whole document, but everytime a new parent is wrapped around it, like:

<div>
    <p class="red">first</p>
    <div class="red">second</div>
</div>
<div>
    <p class="red">third</p>
    <div class="red">fourth</div>
</div>

first and third will be red then.

Update:

I dont know why martyn deleted his answer, but he had the solution, the :nth-of-type selector:

<html>
<head>
<style type="text/css">
.red:nth-of-type(1)
{
    border:5px solid red;
} 
</style>
</head>

<body>
    <div class="home">
        <span>blah</span>
        <p class="red">first</p>
        <p class="red">second</p>
        <p class="red">third</p>
        <p class="red">fourth</p>
    </div>
</body>
</html>

Credits to Martyn. More infos for example here. Be aware that this is a CSS 3 selector, therefore not all browsers will recognize it (eg IE8 or older).

The correct answer is:

.red:first-child, :not(.red) + .red { border:5px solid red }

Part I: If element is first to its parent and has class "red", it shall get border.
Part II: If ".red" element is not first to its parent, but is immediately following an element without class ".red", it shall also deserve the honor of said border.

Fiddle or it didn't happen.

Philip Daubmeier's answer, while accepted, is not correct - see attached fiddle.
BoltClock's answer would work, but unnecessarily defines and overwrites styles
(particularly an issue where it otherwise would inherit a different border - you don't want to declare other to border:none)

EDIT: In the event that you have "red" following non-red several times, each "first" red will get the border. To prevent that, one would need to use BoltClock's answer. See fiddle

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