Is there any impact on performance when specifying div.class or p.class instead of just .class if a certain class will only be used on divs or paragraphs?
Is there any impact on performance when specifying div.class or p.class instead of just .class if a certain class will only be used on divs or paragraphs?
If you want to try this out for yourself, Steve Souders has created a tool called the "CSS Test Creator" that allows you to test the speed of various CSS selectors:
I conducted tests on both .class and a.class using a set of 10,000 rules and anchors each. After running each test 5 times, these were the results obtained:
+----------+-----------+----------+
| Test # | a.class | .class |
+----------+-----------+----------+
| 1 | 2915 ms | 2699 ms |
| 2 | 3021 ms | 2869 ms |
| 3 | 2887 ms | 3033 ms |
| 4 | 2990 ms | 3035 ms |
| 5 | 2987 ms | 2980 ms |
+----------+-----------+----------+
| AVERAGE | 2960 ms | 2923 ms |
+----------+-----------+----------+
The results show that .class is marginally faster, with a difference of just 37ms over 10,000 elements. However, there is a valid reason to opt for .class instead of tag.class, which is flexibility. If you have several <div class="content"> elements that are later changed to <section class="content"> elements, you would need to update your CSS if you used div.content rules. On the other hand, if you used .content, no CSS modifications would be necessary.
In general, I recommend utilizing tag.class style selectors only when dealing with multiple tag types sharing the same class, and you want to specifically target one type of tag.
A speed comparison between .class and element.class reveals that the former is slightly faster. This can be attributed to how CSS is processed from left to right. In essence, .class targets elements with a class name of .class, whereas element.class narrows this down further by specifying elements with both the class name .class and are also of a specific element type.
It is worth noting that although .class may offer quicker matching, element.class holds a higher specificity value.
Although this may not have been the original question, it does play a role in specificity. As shown in the example below, the innerClassFails does not override the contained tds, while the table.innerClass successfully overrides them.
<style>
table.outerClass td {
color: red;
}
table.innerClass td {
color: green;
}
.innerClassFails td {
color: green;
}
</style>
<table class='outerClass'><tr><td><table class='innerClass'><tr><td>Hello!</td></tr></table></td></tr></table>
<table class='outerClass'><tr><td><table class='innerClassFails'><tr><td>Hello!</td></tr></table></td></tr></table>
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