Basic CSS Selectors Tutorial

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1- What is CSS Selector?

CSS Selector is used to search (or select) elements in an HTML document to apply styles to them.
For a simple example:
/** Select all elements with class contains abc */

.abc  

/** Select element with ID = abc */

#abc 
CSS Selectors can be divided into 3 types as follows:

Basic Selectors

Basic Selectors help you select elements based on their Name, ID, Class or Attribute.
The basic Selector will be mentioned in this article.

Combining selectors

Select elements based on the relationship among them.
Combinator Syntax Example
Descendant combinator A B div span
div .item
Child combinator A > B ul > li
General sibling combinator A ~ B p ~ span
Adjacent sibling combinator A + B h2 + p
Column combinator A || B col || td

Pseudo selectors

Pseudo Syntax Example
Pseudo classes A:B a:visited
Pseudo elements A::B p::first-line
  • TODO Link!

2- Universal selector

Universal selector: Select all the elements or all the elements of a namespace.
Syntax:
Selector Description
* Select all the elements.
*|* Select all the elements.
ns|* Select all the elements in a namespace called ns.
|* Select all the elements with an undeclared namespace.
For example, select all the elements and set a blue border for them.
universal-selector-example1.html
<!DOCTYPE html>
<html>
   <head>
      <meta charset="utf-8"/>
      <title>Universal Selector: *</title>
      <style>
         *  {
           border: 1px solid green;
           margin: 5px;
         }
      </style>
   </head>
   <body>

     <h1>I am H1 element</h1>

     <div>I am div element</div>

     <p>I am P element</div>

   </body>
</html>

3- CSS Type Selector

CSS Type Selector helps you find the elements by name.
For example, select all of the  <div> elements:
div  {
   color: red;
}
Another example, select all of the  <span> elements:
type-selector-example1.css
span  {
   background-color: skyblue;
}
type-selector-example1.html
<!DOCTYPE html>
<html>
   <head>
      <meta charset="utf-8"/>
      <title>Universal Selector: ns|*</title>
      <link rel="stylesheet" type="text/css" href="type-selector-example1.css" />
   </head>
   <body>

     <span>I am span element</span>

     <p>I am P element</p>

     <span>I am span element</span>

   </body>
</html>
If CSS selectors have the same rule, you can write them briefly. CSS selectors will be separated by commas and follow the same rule.
type-selector-example2.css
/** Select H1 or H2 */
h1, h2  {
   color: blue;
}
type-selector-example2.html
<!DOCTYPE html>
<html>
   <head>
      <meta charset="utf-8"/>
      <title>Type Selector</title>
      <link rel="stylesheet" type="text/css" href="type-selector-example2.css" />
   </head>
   <body>

     <h1>I am H1 element</h1>

     <h2>I am H2 element</h2>

     <h3>I am H3 element</h3>

   </body>
</html>

4- CSS Class Selector

CSS Class Selector selects elements based on the value of the class attribute.
For example, select all the elements with the  class = "txt-green" or  "txt-green" included.
class-selector-example1.css
.txt-green  {
   color: green;
}
.bg-yellow {
   background-color: yellow;
}
class-selector-example1.html
<!DOCTYPE html>
<html>
   <head>
      <meta charset="utf-8"/>
      <title>Class Selector</title>
      <link rel="stylesheet" type="text/css" href="class-selector-example1.css" />
   </head>
   <body>

     <h1 class ="txt-green">I am H1 with class = 'txt-green'</h1>

     <p>I am P element</p>

     <h2 class="txt-green bg-yellow">I am H2 with class = 'txt-green bg-yellow'</h2>

   </body>
</html>
Example:
class-selector-example2.css
/* <p> with class 'txt-green' or includes 'text-green' */
p.txt-green  {
   color: green;
}

.bg-yellow {
   background-color: yellow;
}

/* <div> with class inculudes both 'text-green' and 'bg-yellow' */
div.txt-green.bg-yellow  {
    color: green;
    font-style: italic;
}
class-selector-example2.html
<!DOCTYPE html>
<html>
   <head>
      <meta charset="utf-8"/>
      <title>Class Selector</title>
      <link rel="stylesheet" type="text/css" href="class-selector-example2.css" />
   </head>
   <body>

     <h1 class ="txt-green">I am H1 with class = 'txt-green'</h1>

     <p class ="txt-green">I am P element with class='txt-green'</p>

     <div class="txt-green bg-yellow">I am DIV with class = 'txt-green bg-yellow'</div>

     <br/>

     <span class="txt-green bg-yellow">I am SPAN with class = 'txt-green bg-yellow'</span>

   </body>
</html>

5- CSS ID Selector

CSS ID Selector helps you select elements based on the value of the ID attribute. The value of the ID attribute must exactly match the value given by Selector. Note: The CSS ID Selector is "Case insensitive".
id-selector-example.css
#demo  {
   color: blue;
   font-size: 30px;
}
id-selector-example.html
<!DOCTYPE html>
<html>
   <head>
      <meta charset="utf-8"/>
      <title>ID Selector</title>
      <link rel="stylesheet" type="text/css" href="id-selector-example.css" />
   </head>
   <body>

     <h1 id = "demo">Demo</h1>

     <span>I am span element</span>

     <p>I am P element</p>

   </body>
</html>

6- CSS Attribute Selector

CSS Attribute Selector helps you select elements based on the attribute or value of a given attribute.

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