<button class="button">CSS A11y Button</button>

<div class="controls">
  <h2 class="title">Button Background Color</h2>
  
  <fieldset>
    <div class="form-element">
      <label for="red">Red</label>
      <input id="red" class="range red" name="red" type="range" min="0" max="255">
    </div>
  </fieldset>
  
  <fieldset>
    <div class="form-element">
      <label for="green">Green</label>
      <input id="green" class="range green" name="green" type="range" min="0" max="255">
    </div>
  </fieldset>
  
  <fieldset>
    <div class="form-element">
      <label for="blue">Blue</label>
      <input id="blue" class="range blue" name="blue" type="range" min="0" max="255">
    </div>
  </fieldset>
</div>

:root {
  --red: 28;
  --green: 150;
  --blue: 130;
  
  --accessible-color: calc(
    (
      (
        (
          (var(--red) * 299) +
          (var(--green) * 587) +
          (var(--blue) * 114)
        ) / 1000
      ) - 128
    ) * -1000
  );
}

html {
  box-sizing: border-box;
}

*,
*::before,
*::after {
  box-sizing: inherit;
  padding: 0;
  margin: 0;
}

body {
  display: flex;
  flex-direction: column;
  height: 100vh;
  font-family: Montserrat, sans-serif;
}

.button {
  appearance: none;
  padding: 0.5em 1em;
  margin: auto;
  color:
    rgb(
      var(--accessible-color),
      var(--accessible-color),
      var(--accessible-color)
    );
  font-size: 1.5em;
  line-height: 1;
  font-family: inherit;
  background-color:
    rgb(
      var(--red),
      var(--green),
      var(--blue)
    );
  border: 0;
  border-radius: 1.5em;
}


/* Controls */

.controls {
  position: relative;
  z-index: 1;
  display: flex;
  flex-wrap: wrap;
  padding: 1rem;
  color: #333;
  text-transform: uppercase;
  background-color: rgba(220,220,220,0.5);
}

.controls .title {
  flex: 2 0 100%;
  margin: 0.5rem;
  font-weight: 900;
  font-size: 1.25em;
  text-align: center;
  letter-spacing: 0.03125em;
}

.controls fieldset {
  width: calc(33.333% - 1rem);
  border: none;
  margin: 0.5rem;
}

.controls label {
  display: block;
  font-size: x-small;
  letter-spacing: 0.0625em;
}

.controls input[type="range"] {
  position: relative;
  width: 100%;
}

.controls input[type="range"]::before {
  content: attr(data-value);
  position: absolute;
  bottom: 100%;
  right: 0;
}

// For interactive purposes only

var styles = getComputedStyle(document.body);
var root = document.documentElement;
var range = document.querySelectorAll('.range');

function setDataVal(element) {
  element.setAttribute('data-value', element.value);
}

for (var i = 0; i < range.length; i++) {
  var rangeItem = range[i];
  var initialRangeValue = parseInt(styles.getPropertyValue('--' + rangeItem.name));
  
  rangeItem.value = initialRangeValue;
  setDataVal(rangeItem);
  
  rangeItem.addEventListener('input', function() {
    root.style.setProperty('--' + this.name, this.value);
    setDataVal(this);
  });
}