Classes in HTML are used to identify specific elements and are defined by the class attribute. Class values are typically used to apply particular styles to the element or to enable particular behaviour, such as JavaScript functions. They can be used on any HTML tag. For example:

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<div class="example">This div has a class name of "example"</div>

A class in JavaScript is a type of object that provides the ability to group properties, methods and functions together into a larger structure. A class can be used to create many instances of the same type of object, and each instance can maintain its own set of data. Classes in JavaScript are defined using the class keyword and can include constructors, static methods and prototype methods.

Example:

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class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  sayHello() {
    console.log(`Hello, my name is ${this.name} and I am ${this.age}`);
  }
}

const person1 = new Person('John', '21');
person1.sayHello(); //prints 'Hello, my name is John and I am 21'

Reference: MDN Web Docs - Classes - JavaScript

The div and span elements are both used to define sections in an HTML document, but they differ in usage. The div element is used to group and structure content, while the span element is used to apply styling and layout to individual sections without defining a new structure. One of the most important differences is that classes can be used with div and span elements to create reusable sections that can be styled using CSS.

For example, a class called "container" could be used to center the HTML content of a page in a div element. This could be accomplished by adding the class name to the header:

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<div class="container">
    <!-- Content goes here -->
</div>

Alternatively, if you only wanted a selected part of your page to be styled or laid out, you could do that in a span element with a class name:

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<span class="myClass">
    <!-- Content goes here -->
</span>

The class name could then be used with CSS to control the styling or layout of the content.

More information can be found in this article on difference between div and span elements.

Yes, you can add multiple classes to a HTML element by separating them with spaces. For example:

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<p class="class1 class2 class3">My paragraph</p>

Reference: https://www.w3schools.com/html/html_classes.asp

Using CSS you can create an effect that makes text appear to be "hovering" on the screen. To do this, you need to create two separate classes, one for when the mouse is hovering over the text (for example, when the mouse is hovering over a link on a page) and one for when the mouse is not hovering over the text. To create the hover effect, you set different values on the text-decoration property between the two classes.

An example of using the :hover effect to underline text is provided here:

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.text {
  text-decoration: none;
}

.text:hover {
  text-decoration: underline;
}

You can create a basic grid system in Bootstrap by building on the default <div class="row"> and <div class="col-*-*"> classes. Specifically, you can use the .col- class on a specified number of elements and Bootstrap will recognize how many elements there are and it will distribute the widths accordingly.

For example, if you want to create a 3x3 grid you can use the following code:

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<div class="row">
  <div class="col-4">Content</div>
  <div class="col-4">Content</div>
  <div class="col-4">Content</div>
  <div class="col-4">Content</div>
  <div class="col-4">Content</div>
  <div class="col-4">Content</div>
  <div class="col-4">Content</div>
  <div class="col-4">Content</div>
  <div class="col-4">Content</div>
</div>

For additional information, feel free to check out the Bootstrap documentation on Grids.

In Bootstrap 4, column width can be adjusted using the classes col-* or col-lg-* for wider columns and col-sm-* for narrower columns.

For example, setting col-sm-4 gives a column width of 33.3333% for extra small devices, and setting col-4 gives a column width of 25% for all device sizes. You can also use fractional numbers (for example, col-6.5).

You can combine these classes to give different column widths per device size, as shown below.

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<div class = "row">
   <div class = "col-12 col-lg-6 col-md-8"> Column 1</div>
   <div class = "col-12 col-lg-6 col-md-4"> Column 2 </div>
</div>

In the example above, Column 1 will be 100% width for extra small devices, 50% width for large devices, and 62.5% width for medium devices, while Column 2 will be 100% width for extra small and medium devices, and 50% for large devices.

For more information, see the docs on the Bootstrap 4 grid system.