You're looking at the documentation for Tailwind CSS v2.
Tailwind CSS on GitHub

Backdrop Opacity

Tailwind CSS version
v2.1+

Utilities for applying backdrop opacity filters to an element.

Default class reference

Class
Properties
backdrop-opacity-0--tw-backdrop-opacity: opacity(0);
backdrop-opacity-5--tw-backdrop-opacity: opacity(0.05);
backdrop-opacity-10--tw-backdrop-opacity: opacity(0.1);
backdrop-opacity-20--tw-backdrop-opacity: opacity(0.2);
backdrop-opacity-25--tw-backdrop-opacity: opacity(0.25);
backdrop-opacity-30--tw-backdrop-opacity: opacity(0.3);
backdrop-opacity-40--tw-backdrop-opacity: opacity(0.4);
backdrop-opacity-50--tw-backdrop-opacity: opacity(0.5);
backdrop-opacity-60--tw-backdrop-opacity: opacity(0.6);
backdrop-opacity-70--tw-backdrop-opacity: opacity(0.7);
backdrop-opacity-75--tw-backdrop-opacity: opacity(0.75);
backdrop-opacity-80--tw-backdrop-opacity: opacity(0.8);
backdrop-opacity-90--tw-backdrop-opacity: opacity(0.9);
backdrop-opacity-95--tw-backdrop-opacity: opacity(0.95);
backdrop-opacity-100--tw-backdrop-opacity: opacity(1);

Usage

Use the backdrop-opacity-{amount} utilities alongside the backdrop-filter utility to control the opacity of other backdrop filters applied to an element.

<div class="backdrop-filter backdrop-opacity-80 ...">
  <!-- ... -->
</div>

Responsive

To control an element’s backdrop opacity at a specific breakpoint, add a {screen}: prefix to any existing backdrop opacity utility. For example, use md:backdrop-opacity-lg to apply the backdrop-opacity-lg utility at only medium screen sizes and above.

<div class="backdrop-filter backdrop-opacity-sm md:backdrop-opacity-lg ...">
  <!-- ... -->
</div>

For more information about Tailwind’s responsive design features, check out the Responsive Design documentation.

Customizing

You can customize which backdrop-opacity utilities are generated using the backdropOpacity key in the theme section of your tailwind.config.js file.

  // tailwind.config.js
  module.exports = {
    theme: {
+     extend: {
+       backdropOpacity: {
+         15: '.15',
+       }
+     }
    }
  }

Learn more about customizing the default theme in the theme customization documentation.

Variants

By default, only responsive variants are generated for backdrop opacity utilities.

You can control which variants are generated for the backdrop opacity utilities by modifying the backdropOpacity property in the variants section of your tailwind.config.js file.

For example, this config will also generate hover and focus variants:

  // tailwind.config.js
  module.exports = {
    variants: {
      extend: {
        // ...
+       backdropOpacity: ['hover', 'focus'],
      }
    }
  }

Disabling

If you don't plan to use the backdrop opacity utilities in your project, you can disable them entirely by setting the backdropOpacity property to false in the corePlugins section of your config file:

  // tailwind.config.js
  module.exports = {
    corePlugins: {
      // ...
+     backdropOpacity: false,
    }
  }