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Selectors Redirect 1

This is the 5th section of the CSS Getting Started tutorial; it explains how you can apply styles selectively, and how different kinds of selectors have different priorities. You add some attributes to the tags in your sample document, and you use these attributes in your sample stylesheet.

Information: Selectors

CSS has its own terminology to describe the CSS language. Previously in this tutorial, you created a line in your stylesheet like this:

strong {
  color: red;
}

In CSS terminology, this entire line is a rule. This rule starts with strong, which is a selector. It selects which elements in the DOM the rule applies to.

More details

The part inside the curly braces is the declaration.

The keyword color is a property, and red is a value.

The semicolon after the property-value pair separates it from other property-value pairs in the same declaration.

This tutorial refers to a selector like strong as a tag selector. The CSS Specification refers to it as a type selector.

This page of the tutorial explains more about the selectors that you can use in CSS rules.

In addition to tag names, you can use attribute values in selectors. This allows your rules to be more specific.

Two attributes have special status for CSS. They are class and id.

Class selectors

Use the class attribute in an element to assign the element to a named class. It is up to you what name you choose for the class. Multiple elements in a document can have the same class value.

In your stylesheet, type a full stop (period) before the class name when you use it in a selector.

ID selectors

Use the id attribute in an element to assign an ID to the element. It is up to you what name you choose for the ID. The ID name must be unique in the document.

In your stylesheet, type a number sign (hash) before the ID when you use it in a selector.

Example
This HTML tag has both a class attribute and an id attribute:
<p class="key" id="principal">

The id value, principal, must be unique in the document, but other tags in the document can have the same class name, key.

In a CSS stylesheet, this rule makes all the elements with class key green. (They might not all be <p> elements.)

.key {
  color: green;
}

This rule makes the one element with the id principal bold:

#principal {
  font-weight: bolder;
}

If more than one rule applies to an element and specifies the same property, then CSS gives priority to the rule that has the more specific selector. An ID selector is more specific than a class selector, which in turn is more specific than a tag selector.

More details

You can also combine selectors, making a more specific selector.

For example, the selector .key selects all elements that have the class name key. The selector p.key selects only <p> elements that have the class name key.

You are not restricted to the two special attributes, class and id. You can specify other attributes by using square brackets. For example, the selector [type='button'] selects all elements that have a type attribute with the value button.

If the stylesheet has conflicting rules and they are equally specific, then CSS gives priority to the rule that is later in the stylesheet.

When you have a problem with conflicting rules, try to resolve it by making one of the rules more specific, so that it has priority. If you cannot do that, try moving one of the rules nearer the end of the stylesheet so that it has priority.

Pseudo-classes selectors

A CSS pseudo-class is a keyword added to selectors that specifies a special state of the element to be selected. For example :hover will apply a style when the user hovers over the element specified by the selector.

Pseudo-classes, together with pseudo-elements, let you apply a style to an element not only in relation to the content of the document tree, but also in relation to external factors like the history of the navigator (:visited, for example), the status of its content (like :checked on some form elements), or the position of the mouse (like :hover which lets you know if the mouse is over an element or not). To see a complete list of selectors, visit CSS3 Selectors working spec.

Syntax
selector:pseudo-class {
  property: value;
} 

List of pseudo-classes

Information: Selectors based on relationships

CSS has some ways to select elements based on the relationships between elements. You can use these to make selectors that are more specific.

Common selectors based on relationships
Selector Selects
A E Any E element that is a descendant of an A element (that is: a child, or a child of a child, etc.)
A > E Any E element that is a child of an A element
E:first-child Any E element that is the first child of its parent
B + E Any E element that is the next sibling of a B element (that is: the next child of the same parent)

You can combine these to express complex relationships.

You can also use the symbol * (asterisk) to mean "any element".

Example

An HTML table has an id attribute, but its rows and cells do not have individual identifiers:

<table id="data-table-1">
...
<tr>
<td>Prefix</td>
<td>0001</td>
<td>default</td>
</tr>
...

These rules make the first cell in each row bold, and the second cell in each row monospaced. They only affect one specific table in the document:

    #data-table-1 td:first-child {font-weight: bolder;}
    #data-table-1 td:first-child + td {font-family: monospace;}

The result looks like:

Prefix 0001 default
More details

In the usual way, if you make a selector more specific, then you increase its priority.

If you use these techniques, you avoid the need to specify class or id attributes on so many tags in your document. Instead, CSS does the work.

In large designs where speed is important, you can make your stylesheets more efficient by avoiding complex rules that depend on relationships between elements.

For more examples about tables, see Tables in the CSS Reference page.

Action: Using class and ID selectors

  1. Edit your HTML file, and duplicate the paragraph by copying and pasting it.
  2. Then add id and class attributes to the first copy, and an id attribute to the second copy as shown below. Alternatively, copy and paste the entire file again:
    <!doctype html>
    <html>
      <head>
      <meta charset="UTF-8">
      <title>Sample document</title>
      <link rel="stylesheet" href="style1.css">
      </head>
      <body>
        <p id="first">
          <strong class="carrot">C</strong>ascading
          <strong class="spinach">S</strong>tyle
          <strong class="spinach">S</strong>heets
        </p>
        <p id="second">
              <strong>C</strong>ascading
              <strong>S</strong>tyle
              <strong>S</strong>heets
            </p>
      </body>
    </html>
    
  3. Now edit your CSS file. Replace the entire contents with:
    strong { color: red; }
    .carrot { color: orange; }
    .spinach { color: green; }
    #first { font-style: italic; }
    
  4. Save the files and refresh your browser to see the result:
    Cascading Style Sheets
    Cascading Style Sheets

    You can try rearranging the lines in your CSS file to show that the order has no effect.

    The class selectors .carrot and .spinach have priority over the tag selector strong.

    The ID selector #first has priority over the class and tag selectors.

Challenges
  1. Without changing your HTML file, add a single rule to your CSS file that keeps all the initial letters that same color as they are now, but makes all the other text in the second paragraph blue:
    Cascading Style Sheets
    Cascading Style Sheets
  2. Now change the rule you have just added (without changing anything else), to make the first paragraph blue too:
    Cascading Style Sheets
    Cascading Style Sheets
Possible solution
  1. Add a rule with an ID selector of #second and a declaration color: blue;, as shown below:
    #second { color: blue; }
    
    A more specific selector, p#second also works.
  2. Change the selector of the new rule to be a tag selector using p:
    p { color: blue; }
    
Hide solution
See a solution for the challenge.

Action: Using pseudo-classes selectors

  1. Create an HTML like the following:
    <!doctype html>
    <html>
      <head>
      <meta charset="UTF-8">
      <title>Sample document</title>
      <link rel="stylesheet" href="style1.css">
      </head>
      <body>
        <p>Go to our <a class="homepage" href="http://www.example.com/" title="Home page">Home page</a>.</p>
      </body>
    </html>
    
  2. Now edit your CSS file. Replace the entire contents with:
    a.homepage:link, a.homepage:visited {
      padding: 1px 10px 1px 10px;
      color: #fff;
      background: #555;
      border-radius: 3px;
      border: 1px outset rgba(50,50,50,.5);
      font-family: georgia, serif;
      font-size: 14px;
      font-style: italic;
      text-decoration: none;
    }
    
    a.homepage:hover, a.homepage:focus, a.homepage:active {
      background-color: #666;
    }
    
  3. Save the files and refresh your browser to see the result (put the mouse over the following link to see the effect):
    Go to our Home page  

Action: Using selectors based on relationships and pseudo-classes

With selectors based on relationships and pseudo-classes you can create complex cascade algorithms. This is a common technique used, for example, in order to create pure-CSS dropdown menus (that is only CSS, without using JavaScript). The essence of this technique is the creation of a rule like the following:

div.menu-bar ul ul {
  display: none;
}

div.menu-bar li:hover > ul {
  display: block;
}

to be applied to an HTML structure like the following:

<div class="menu-bar">
  <ul>
    <li>
      <a href="example.html">Menu</a>
      <ul>
        <li>
          <a href="example.html">Link</a>
        </li>
        <li>
          <a class="menu-nav" href="example.html">Submenu</a>
          <ul>
            <li>
              <a class="menu-nav" href="example.html">Submenu</a>
              <ul>
                <li><a href="example.html">Link</a></li>
                <li><a href="example.html">Link</a></li>
                <li><a href="example.html">Link</a></li>
                <li><a href="example.html">Link</a></li>
              </ul>
            </li>
            <li><a href="example.html">Link</a></li>
          </ul>
        </li>
      </ul>
    </li>
  </ul>
</div>

See our complete CSS-based dropdown menu example for a possible cue.

What next?

Your sample stylesheet is starting to look dense and complicated. The next section describes ways to make CSS easier to read.

Document Tags and Contributors

Contributors to this page: Sheppy
Last updated by: Sheppy,