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                <!-- Include A-Frame latest master distribution, via RawGit. -->
<!-- Really should be in <head>; in CodePen body for simple viewing. -->
<script src=""></script>

<!-- Components and shaders go after A-Frame, but before the scene declaration. -->
AFRAME.registerShader('my-custom', {
  // The schema declares any parameters for the shader.
  schema: {
    // Where relevant, it is customary to support color.
    // The A-Frame schema uses `type:'color'`, so it will parse string values like 'white' or 'red.
    // `is:'uniform'` tells A-Frame this should appear as uniform value in the shader(s).
    color: {type:'color', is:'uniform', default:'red'},
    // It is customary to support opacity, for fading in and out.
    opacity: {type:'number', is:'uniform', default:1.0}
  // Setting raw to true uses THREE.RawShaderMaterial instead of ShaderMaterial,
  // so your shader strings are used as-is, for advanced shader usage.
  // Here, we want the usual prefixes with GLSL constants etc.,
  // so we set it to false.
  // (Which is also the default, so we could have omitted it).
  raw: false,
  // Here, we're going to use the default vertex shader by omitting vertexShader.
  // But note that if your fragment shader cares about texture coordinates,
  // the vertex shader should set varying values to use in the fragment shader. 
  // Since almost every WebVR-capable browser supports ES6,
  // define our fragment shader as a multi-line string.
  // Use medium precision.
  precision mediump float;

  // This receives the color value from the schema, which becomes a vec3 in the shader.
  uniform vec3 color;

  // This receives the opacity value from the schema, which becomes a number.
  uniform float opacity;

  // This is the shader program.
  // A fragment shader can set the color via gl_FragColor,
  // or decline to draw anything via discard.
  void main () {
    // Note that this shader doesn't use texture coordinates.
    // Set the RGB portion to our color,
    // and the alpha portion to our opacity.
    gl_FragColor = vec4(color, opacity);

<!-- Declaration of the A-Frame scene. -->
  <!-- Black skybox at effectively infinite distance. -->
  <!-- a-box is used instead of a-sky because it requires less processing than the sphere that a-sky generates. -->
  <!-- Z scale is inverted so that the inside surface gets the color. -->
  <a-box width="10000" height="10000" depth="-10000" color="black">
    <!-- Inside the black skybox, a box using our shader, 2 meters ahead, not fully opaque, and blue. -->
    <a-box material="shader:my-custom; color:blue; opacity:0.7; transparent:true" position="0 0 -2"></a-box>
    <!-- Inside the black skybox, a box behind the previous one, 3 meters ahead. -->
    <a-box position="0 0 -3"></a-box>