<header class="header">
  <div class="header__title">
    <h1>
      <code>brightness()</code>
    </h1>
  </div>
  <div class="header__reference">
    <ul class="reference-list">
      <li><a class="reference-list__link" href="https://developer.mozilla.org/en-US/docs/Web/CSS/filter-function/brightness">MDN reference</a></li>
      <li><a class="reference-list__link" href="https://caniuse.com/#feat=css-filters">caniuse Support</a></li>
    </ul>
  </div>
</header>

<main>
  <div class="example">
    <p>
      In this example, brightness is set to <code>50%</code> using a CSS Custom Property. By moving the range slider to either the left or right, you can increase or decrease the brightness effect applied to the photo.
    </p>
    
    <div class="controls">
      <label class="controls__label" for="brightness">Brightness:</label>
      <input class="controls__input" type="range" id="brightness" min="0" max="100" value="50">
    </div>

    <div class="example__demo example__brightness">
      <img class="drink" alt="A Manhattan cocktail placed on a wooden table in a low-lit room." src="https://s3-us-west-2.amazonaws.com/s.cdpn.io/11907/jakub-dziubak-gj7BLlSzIFs-unsplash.jpg"/>
    </div>
  </div>
  
  <aside>
    <p>
      <small>Photo by <a href="https://unsplash.com/photos/gj7BLlSzIFs">Jakub Dziubak</a> on <a href="https://unsplash.com/">Unsplash</a>.</small>
    </p>
  </aside>
</main>
// Demo
:root {
  --brightness: 50%;
}

.drink {
  filter: brightness(var(--brightness));
}


// Pen Setup
.example__brightness {
  background-color: var(--color-cinder);
  padding: 2rem 2rem 1.75rem 2rem;
  
  img {
    height: auto;
    width: 100%;
  }
}

.controls {
  display: flex;
  flex-direction: column;
  margin-top: var(--size-epsilon);
}

.controls__label {
  font-weight: 700;
  letter-spacing: 0.05em;
  text-transform: uppercase;
}

.controls__input {
  margin-top: 0.5rem;
}
View Compiled
// Credit: 
// Update CSS Variables with JS
// Wes Bos
// https://codepen.io/wesbos/pen/adQjoY

const rangeSlider = [].slice.call(document.querySelectorAll('#brightness'));

// Listen for changes
rangeSlider.forEach(input => input.addEventListener('change', handleUpdate));
rangeSlider.forEach(input => input.addEventListener('mousemove', handleUpdate));

function handleUpdate(e) {
  // Append '%' to the end of the `blur` custom property
  const suffix = (this.id === 'base' ? '' : '%');
  document.documentElement.style.setProperty(`--${this.id}`, this.value + suffix);
}

External CSS

  1. https://codepen.io/ericwbailey/pen/vMXWgz

External JavaScript

This Pen doesn't use any external JavaScript resources.