Understanding CSS selector priority / specificity

Question

I'd like to understand how CSS selectors work with property collisions. How is one property selected over another one?

div {
  background-color: red;
}

div.my_class {
  background-color: black;
}

div#my_id {
  background-color: blue;
}

body div {
  background-color: green;
}

body>div {
  background-color: orange;
}

body>div#my_id {
  background-color: pink;
}

<div id="my_id" class="my_class">hello</div>

How does selector priority work?

Drayman

w3.org/TR/CSS2/cascade.html#specificity – Pinky

Answer 1

I'll just toss in a link to the CSS 2.1 spec itself, and how browsers are supposed to calculate specificity:

CSS 2.1 Section 6.4.3:

A selector's specificity is calculated as follows:

• count 1 if the declaration is from is a 'style' attribute rather than a rule with a selector, 0 otherwise (= a) (In HTML, values of an element's "style" attribute are style sheet rules. These rules have no selectors, so a=1, b=0, c=0, and d=0.)
• count the number of ID attributes in the selector (= b)
• count the number of other attributes and pseudo-classes in the selector (= c)
• count the number of element names and pseudo-elements in the selector (= d)
• The specificity is based only on the form of the selector. In particular, a selector of the form "[id=p33]" is counted as an attribute selector (a=0, b=0, c=1, d=0), even if the id attribute is defined as an "ID" in the source document's DTD.

Concatenating the four numbers a-b-c-d (in a number system with a large base) gives the specificity.

If the specificities are equal, then CSS 2.1 Section 6.4.1 comes into play:

4. Finally, sort by order specified: if two declarations have the same weight, origin and specificity, the latter specified wins. Declarations in imported style sheets are considered to be before any declarations in the style sheet itself.

Note that this is talking about when the style is defined, not when it is used. If classes .a and .b have equal specificity, whichever is defined last in the stylesheet(s) wins. <p class="a b">...</p> and <p class="b a">...</p> will be styled identically, based on the definition order of .a and .b.

Hormuz

The best explanation so far: link It starts to become amazingly interesting when you scroll down to the "Calculating CSS Specificity Value" part. – Capsize

But what happens when both specificity values are the same? Does it go by which rule was defined first? – Congruency

That's covered by w3.org/TR/CSS2/cascade.html#cascading-order, specifically rule 4, which applies after specificity. I'll edit the answer to include the tie-breaking. – Hormuz

@Congruency : If there are multiple same specificity rules, then the last rule occuring in the stylesheet is applied. Hope this helps. – Elburr

Answer 2

1. Element
2. Class selectors
3. ID Selectors
4. Inline styles
5. !important

In order, 1 is the lowest specificity and 5 is the highest. https://youtu.be/NqDb9GfMXuo will shown details to demo it.

Flowered

Answer 3

What you are interested in is specificity.

Firebug is a great tool to help inspect this. But other browsers also have built in tools for inspecting the applied CSS rules.

Litmus

Does Firebug have specific (no pun intended) tools to analyze specificity? More than the strike-through view of overridden rules? – Octane

@Pekka, no it does not. Still an excellent tool for these types of things though. – Litmus

@Octane In the right side of HTML tab, the Calculated (?) tab (between Style and Appearance) has another strike-through view of overriden instructions. More useful than the one in Style you're probably referring to when you're interested by only ONE property. – Cantrip

Answer 4

You can refer the full answer here Mozilla documentation

Start from the most specific: id selectors > class selectors > type selectors(normal h1, p tag and so on..) !important always wins, but it is considered a bad practice.See the link above.

The best way is to experiment with it:

  <!-- start class vs id -->
  <p class="class1" id="id1">.class vs #id: The winner is #id</p>

  <!-- upper vs bottom -->
  <p id="id2">the very bottom is the winner</p>

  <!--most specific is the winner -->
  <p id="id3">the most specific</p>

  <!--pseudo and target selector -->
  <h3>pseudo vs type selector</h3>

  <!-- !important is more important! -->
  <h1 id="very-specific">HI! I am very important!</h1>
  </body>

CSS:

#id1{
    color: blue;
}

.class1{
    color: red;
}

#id2{
    color: yellow;
}

#id2{
    color : green;
}

body p#id3{
    color :orange;
}

body p{
    color : purple;
}

body{
    color : black;
}

h3::first-letter {
    color: #ff0000;

}

h3{
    color: CornflowerBlue ; 
}

h1{
    color: gray !important;
}

body h1#very-specific{
    color: red;
}

Here's a test case.

Johnsten