How to get different class selectors using nth-child in css - css

Question

I have these codes
<div class="test">
<div class="tag1"></div>
<div class="tag1"></div>
<div class="tag1"></div>
<div class="tag2"></div>
<div class="tag2"></div>
<div class="tag2"></div>
</div>
Now I want to get second child of .tag2 selector.
I try this code and it's not working, but when I use .tag1 it's working.
.test .tag2:nth-child(2) {
background-color: #000;
}
How can i fix this?

Answer 1

:nth-child works on elements, not on other selectors. Here your .tag2 element is the 4th element in the list.
When browsers begin to implement the Selectors Level 4 standards we'll be able to achieve this using the :nth-match structural pseudo-class selector, but unfortunately that's quite a way off yet.
A Potential CSS Solution (Markup-dependant)
If your markup will always be that the first .tag2 will only ever follow .tag1 and the second .tag2 will only ever follow .tag2, you can fake it with this:
.tag1 + .tag2 + .tag2 {
background-color: #000;
}
This selects the .tag2 element which immediately follows a .tag2 element which immediately follows a .tag1 element.
A JavaScript Solution
If you can't do that then you'll have to go for a JavaScript solution instead (or implement something on the back-end which generates the content).
The below example pulls all .tag2 elements within your .test container, then grabs the 2nd one ([1] here, remember the 0 index: [1] = 2nd), then applies the style to that element.
You'll need to add in some checks to ensure this element exists before applying the style.
document.querySelector('.test').querySelectorAll('.tag2')[1].style.background = '#000'
<div class="test">
<div class="tag1">tag1</div>
<div class="tag1">tag1</div>
<div class="tag1">tag1</div>
<div class="tag2">tag2</div>
<div class="tag2">tag2</div>
<div class="tag2">tag2</div>
</div>

Answer 2

I know your question isn't tagged with JavaScript, but a good solution with JS is as follows:
var alltagtwos = document.getElementsByClassName("tag2");
alltagtwos[1].className += " secondel";
.tag2.secondel {
background-color: #000;
color: #fff;
}
<div class="test">
<div class="tag1">tag1 - 1</div>
<div class="tag1">tag1 - 2</div>
<div class="tag1">tag1 - 3</div>
<div class="tag2">tag2 - 1</div>
<div class="tag2">tag2 - 2</div>
<div class="tag2">tag2 - 3</div>
</div>
What I've done here is add all elements with the class .tag2 into an array-like object using getElementsByClassName. I then select the second element which has the class .tag2 (index starts at zero, so I've select [1]) and appended another class to it (.secondel) which I've then styled with CSS.