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A selector allows us to associate one or more declarations to one or more elements on the page.

Basic selectors

Suppose we have a p element on the page, and we want to display the words into it using the yellow color.

We can target that element using this selector p, which targets all the element using the p tag in the page. A simple CSS rule to achieve what we want is:

p {
  color: yellow;
}

Every HTML tag has a corresponding selector, for example: div, span, img.

If a selector matches multiple elements, all the elements in the page will be affected by the change.

HTML elements have 2 attributes which are very commonly used within CSS to associate styling to a specific element on the page: class and id.

There is one big difference between those two: inside an HTML document you can repeat the same class value across multiple elements, but you can only use an id once. As a corollary, using classes you can select an element with 2 or more specific class names, something not possible using ids.

Classes are identified using the . symbol, while ids using the # symbol.

Example using a class:

<p class="dog-name">
  Roger
</p>
.dog-name {
  color: yellow;
}

Example using an id:

<p id="dog-name">
  Roger
</p>
#dog-name {
  color: yellow;
}

Combining selectors

So far we’ve seen how to target an element, a class or an id. Let’s introduce more powerful selectors.

Targeting an element with a class or id

You can target a specific element that has a class, or id, attached.

Example using a class:

<p class="dog-name">
  Roger
</p>
p.dog-name {
  color: yellow;
}

Example using an id:

<p id="dog-name">
  Roger
</p>
p#dog-name {
  color: yellow;
}

Why would you want to do that, if the class or id already provides a way to target that element? You might have to do that to have more specificity. We’ll see what that means later.

Targeting multiple classes

You can target an element with a specific class using .class-name, as you saw previously. You can target an element with 2 (or more) classes by combining the class names separated with a dot, without spaces.

Example:

<p class="dog-name roger">
  Roger
</p>
.dog-name.roger {
  color: yellow;
}

Combining classes and ids

In the same way, you can combine a class and an id.

Example:

<p class="dog-name" id="roger">
  Roger
</p>
.dog-name#roger {
  color: yellow;
}

Grouping selectors

You can combine selectors to apply the same declarations to multiple selectors. To do so, you separate them with a comma.

Example:

<p>
  My dog name is:
</p>
<span class="dog-name">
  Roger
</span>
p, .dog-name {
  color: yellow;
}

You can add spaces in those declarations to make them more clear:

p,
.dog-name {
  color: yellow;
}

Follow the document tree with selectors

We’ve seen how to target an element in the page by using a tag name, a class or an id.

You can create a more specific selector by combining multiple items to follow the document tree structure. For example, if you have a span tag nested inside a p tag, you can target that one without applying the style to a span tag not included in a p tag:

<span>
  Hello!
</span>
<p>
  My dog name is:
  <span class="dog-name">
    Roger
  </span>
</p>
p span {
  color: yellow;
}

See how we used a space between the two tokens p and span.

This works even if the element on the right is multiple levels deep.

To make the dependency strict on the first level, you can use the > symbol between the two tokens:

p > span {
  color: yellow;
}

In this case, if a span is not a first children of the p element, it’s not going to have the new color applied.

Direct children will have the style applied:

<p>
  <span>
    This is yellow
  </span>
  <strong>
    <span>
      This is not yellow
    </span>
  </strong>
</p>

Adjacent sibling selectors let us style an element only if preceded by a specific element. We do so using the + operator:

Example:

p + span {
  color: yellow;
}

This will assign the color yellow to all span elements preceded by a p element:

<p>This is a paragraph</p>
<span>This is a yellow span</span>

We have a lot more selectors we can use:

  • attribute selectors
  • pseudo-class selectors
  • pseudo-element selectors

More on those on future posts.


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