In CSS3, you can use the box-shadow property to add a shadow to an element. The box-shadow property takes several values that control the appearance of the shadow, including the shadow’s color, size, and position.

Here’s an example of how you might use the box-shadow property to add a shadow to an element:

.my-element {
    box-shadow: 10px 10px 5px #888888;
}

This would add a shadow to the element with the class my-element, with the shadow positioned 10 pixels to the right and 10 pixels below the element, and with a blur radius of 5 pixels. The shadow’s color is specified using the #888888 value, which is a shade of gray.

You can also specify multiple shadows for an element by separating each shadow definition with a comma. For example:

.my-element {
    box-shadow: 10px 10px 5px #888888,
    -10px -10px 5px #888888;
}

This would add two shadows to the element with the class my-element, with one shadow positioned to the right and below the element and the other shadow positioned to the left and above the element.

The box-shadow property is supported in most modern browsers, but it is not supported in older versions of Internet Explorer. If you need to support older browsers, you can use the -moz-box-shadow, -webkit-box-shadow, and -ms-box-shadow vendor-specific prefixes to ensure that the shadow is displayed correctly in all browsers.