Feeling lost when it comes to CSS selectors

Alright, I need some help with this code snippet:

<div id="carousel">
    <div class="active"><img id="pic1" src="http://i.imgur.com/gWGqZly.png" /></div>
    <div><img id="pic2" src="http://i.imgur.com/mC1FD81.png" /></div>
    <div><img id="pic3" src="http://i.imgur.com/HFx9mqa.png" /></div>
</div>

The first div has the class "active" and that's the one I want to target. These divs are stacked on top of each other using position: absolute;

This is my CSS setup:

#carousel div {
    position:absolute;
    z-index: 0;
}
#carousel div.inactive {
    z-index: 1;
}
#carousel div.active {
    z-index: 2;
}

I'm attempting to assign a z-index of "2" to the first div with the class "active". The selector I thought would work is:

.active {
   z-index: 2;
}

However, this doesn't seem to be effective as the image does not come to the front. Surprisingly, if I use this selector instead:

#carousel div.active {
   z-index: 2;
}

It works perfectly.

This got me confused because aren't those two selectors essentially doing the same thing? What sets them apart in this scenario?

You can view a demo on jsfiddle https://jsfiddle.net/x1L4tfw4/5/. If you try removing the "#carousel div" part from the css selector, you'll notice the difference.

Answer №1

Make sure to include the #slider div selector in your CSS file too. This will take precedence over the .current selector due to its higher specificity.

Answer №2

The problem here lies in specificity. The selector #slider div carries a specificity of 101, while #.current only has a specificity of 10.

When combined as #slider div.current, the specificity reaches 111.

So how are these numbers determined?

According to CSS standards, you assign values based on different factors: tag names count as one point, class names or attribute values count as ten points, and ID names count as 100 points (with !important declarations worth 1000 points). By adding up these values, the rule with the highest total is applied. If two rules have equal specificity, the one that comes later in the source code takes precedence.

  • In essence, one ID is more specific than multiple classes due to an infinitely large base system (although browsers actually use base 256).

Answer №3

Your approach to writing CSS can be a bit perplexing. It's important to remember that CSS follows a straightforward system for identifying elements with IDs and classes. While JavaScript places importance on IDs, browsers also have specific advantages when it comes to handling them. However, CSS doesn't prioritize IDs and classes in the same way. This could lead to confusion if not approached carefully.

Avoid using #id element .class in your stylesheet if you have multiple instances of the same <element-tag> in your markup. Doing so may result in style inconsistencies.

Answer №4

The reason behind this issue lies in the CSS specificity of the selectors being used, or potentially due to the inclusion of a third-party library. For a comprehensive understanding of how CSS specificity and inheritance function, check out this informative resource.

Key Insights

An excerpt from the linked article highlights the relationship between different CSS selectors:

  • Element, Pseudo Element: d = 1 – (0,0,0,1)
  • Class, Pseudo class, Attribute: c = 1 – (0,0,1,0)
  • Id: b = 1 – (0,1,0,0)
  • Inline Style: a = 1 – (1,0,0,0)

For further clarity on how this system operates, refer to the examples provided below:

  • p: 1 element – (0,0,0,1)
  • div: 1 element – (0,0,0,1)
  • #sidebar: 1 id – (0,1,0,0)
  • div#sidebar: 1 element, 1 id – (0,1,0,1)
  • div#sidebar p: 2 elements, 1 id – (0,1,0,2)
  • div#sidebar p.bio: 2 elements, 1 class, 1 id – (0,1,1,2)

Applying to Your Case

In your specific scenario, the first selector utilized is .current, with a specificity ranking as follows:

.current (0,0,1,0)

Mentioned by @Admir Geri, another selector present is #slider div which holds the following specificity value:

#slider div (0,1,0,1)

Due to the higher specificity rating of the second selector compared to the first, it takes precedence, thereby resulting in your changes not reflecting.

Your final selector #slider div.current possesses the subsequent CSS specificity:

#slider div.current (0,1,1,1)

Given that this particular score surpasses any other selector's specificity level, your modifications will be visible when using this selector, hence why they are appearing on the screen.

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