CSS - Pseudo-element

CSS - Pseudo-element - ::backdrop

The ::backdrop pseudo-element in CSS is used to create a backdrop that entirely covers the viewport and is displayed or rendered immediately below a <dialog> or an element that enters fullscreen mode or are on the top layer.

The effect of this pseudo-element can be seen in the following scenarios:

All the elements that are placed in fullscreen mode.
All the <dialog> elements that are rendered in the top layer.
All the popover elements that are rendered in the top layer.

Overview

In case of multiple elements placed into the top layer, each element has its own ::backdrop pseudo-element.
The ::backdrop pseudo-element has the ability to conceal, style or make everything completely hidden below the top layer element.
The ::backdrop pseudo-element can not be inherited from or by any other element.
There are no restrictions made on any or all properties that apply on this pseudo-element.

Syntax

selector::backdrop {
   /* ... */
}

CSS ::backdrop Example

Here is an example showing usage of ::backdrop pseudo-element on a <dialog> element:

<html>
<head>
<style>
    dialog::backdrop {
        background-image: url("images/border.png");
    }

    * {
        box-sizing: border-box;
    }
    body {
        margin: 0;
        font-family: 'Lucida Sans', 'Lucida Sans Regular', 'Lucida Grande', 'Lucida Sans Unicode', Geneva, Verdana, sans-serif;
        background-color: #3d3e3e;
        color: white;
    }
    .container {
        max-width: 100%;
        margin: 0 auto;
        padding: 2rem;
    }
    button {
        display: inline-block;
        border: 1px solid #007bff;
        padding: 5px;
        font-size: 1rem;
        color: black;
        background-color: #bfc2c5;
        cursor: pointer;
    }
    @supports not selector(::backdrop) {
        body::before {    
        box-sizing: border-box;
        content: '';
        }
    }
</style>
</head>
<body>
    <div class="container">
        <p>pseudo-element backdrop used to create a background</p>
        <button onclick="openDialog()">Click to Open dialog</button>
        <dialog>
            <p>See the backdrop</p>
            <button onclick="closeDialog()">Close</button>
        </dialog>  
    </div>
    <script>
        var dialog = document.querySelector('dialog');
        function openDialog() {  
        dialog.showModal();
        }
        function closeDialog() {  
        dialog.close();
        }
    </script>
</body>
</html>