Exploring the efficiency of different CSS selectors

I am struggling with the following code:

HTML:

<ul class="list">
    <li class="list-item"> item 1 </li>
    <li class="list-item list-item_active"> Active item </li>    
    <li class="list-item"> item 3 </li>
    <li class="list-item"> item 4 </li>
    <li class="list-item"> item 5 </li>    
</ul>

CSS:

.list-item:nth-child(even) {
    background: #eee
}
.list-item_active {
    background: none;
}

Check out the jsfiddle link - http://jsfiddle.net/Re3xV/2/

Let's assume that .list-item does not have any child elements that can be styled.

Issue : I need .list-item_active to take precedence over .list-item:nth-child(even)

I am trying to figure out which of the following solutions is the fastest in terms of selector performance:

  1. ul .list-item_active
  2. li.list-item_active
  3. .list .list-item_active
  4. .list-item.list-item_active
  5. .list-item_active:nth-child(n)
  6. .list-item:nth-child(even):not(.list-item_active)
  7. .list-item[data-state="active"] (data-state="active" should be added to HTML)

I am considering using 

.list-item_active {
    background: none !important;
}

since it does not seem to impact performance significantly (although I am aware that using !important is generally not recommended). However, I still want to determine which selector is the most efficient. Is there a way to automate such comparisons?

performancecsscss-selectors

Answer №1

The key factor here is the concept of specificity, not necessarily speed. In this scenario, the selector .list-item:nth-child(even) holds a higher level of specificity compared to .list-item_active.

Replace:

.list-item_active { ... }

With:

.list-item.list-item_active { ... }

Check out this example on JSFiddle for more insight: http://jsfiddle.net/JamesD/Re3xV/3/

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