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HTML

              
                <canvas id="rainbow"></canvas>

              
            
!

CSS

              
                body {
  background: #13142b;
}

canvas {
  opacity: .6;
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}

              
            
!

JS

              
                // VARS
const DPR = window.devicePixelRatio
const colors = [
  ['#EC008C', '#f957b6'],
  ['#EF4136', '#ff7972'],
  ['yellow', '#fff'],
  ['lime', '#7aff7a'],
  ['#27AAE1', '#5ec8f2'],
  ["#662D91", '#a158d8']
]
const tau = Math.PI * 2
const start = Math.PI // Start position
const finish = .5 // Finish (in % of circle "tau" basically ending at Math.PI * 2)
const inc = .007
const rainbowHeight = .5 // of view height
const arcStagger = .05 // in %
const sparklesInPerStripe = 3

let sparkles = []

// UTILS
const clamp = (min, max, val) => {
  return Math.min(Math.max(min, val), max)
}
const boolRandom = () => {
  return Math.round(Math.random()) ? false : true
}

// CANVAS
const sizeCanvas = () => {
  radius = clamp(15, 50, window.innerWidth / 60 / DPR)
  const canvas = document.getElementById('rainbow')
  canvas.width = window.innerWidth * DPR
  canvas.height = window.innerHeight * DPR
}

// SPARKLE PROPS
const addRandom = function (lineWidth) {
  return (boolRandom() ? -1 : 1) * Math.random() * lineWidth
}
const makeSparkle = ({cx, cy, radiusX, radiusY, endAngle, lineWidth, color}) => {
  return {
    x: cx + radiusX * Math.cos(endAngle) + addRandom(lineWidth), // stay out in front
    y: cy + radiusY * Math.sin(endAngle) + addRandom(lineWidth), 
    opacity: 1, 
    color, 
    rad: Math.max(radius * Math.random() * DPR, 15)
  }
}

// ANIMATE
function animate(percent = 0) {
  const doneAnimatingIn = percent >= finish + arcStagger * colors.length // animating in rainbow arcs
  
  let width = window.innerWidth * DPR
  let height = window.innerHeight * DPR
  
  const lineWidth = height * .5 / colors.length
  
  const cx = width / 2
  const startCy = height + lineWidth * rainbowHeight + (height - colors.length * lineWidth) / 3 // Possibly simplify this... but the math is good :) 
  
  const startRadiusX = width / 2 + colors.length * lineWidth * 2
  const startRadiusY = height

  let ctx = document.getElementById('rainbow').getContext('2d')
  ctx.clearRect(0, 0, width, height);  
  ctx.globalAlpha = 1;
  ctx.lineWidth = lineWidth
  
  for (let i = colors.length - 1; i > -1; i--) {
    const [colorLine, colorSparkle] = colors[i]
    
    const cy = startCy + i * (lineWidth / 2 - 1) // - 1 for overlap
    
    // Making these "concentric" ellipses
    const radiusX = startRadiusX - i * lineWidth / 2
    const radiusY = startRadiusY - i * lineWidth / 2
    
    const endAngle = tau * (percent - i * arcStagger) + start
    
    const angle = clamp(start, tau * finish + start, endAngle)
    
    // DRAW ONE OF OUR ELLIPSE LINES
    // - One color of our rainbow
    ctx.beginPath()
    ctx.shadowColor = colorLine
    ctx.strokeStyle = colorLine
    ctx.ellipse(
      cx,
      cy,
      radiusX,
      radiusY,
      0,
      start,
      angle,
      false
    )
    ctx.lineCap = "round"
    ctx.stroke()
    ctx.closePath()    
    
    
    if (!doneAnimatingIn) { 
      // ADD: Animating in sparkles, follow the start of each color
      for (let j = 0; j < sparklesInPerStripe; j++) {
        sparkles.push(
          makeSparkle({cx, cy, radiusX, radiusY, endAngle: angle, lineWidth, color: colorLine})
        )
      }
    } else {
      // ADD: Normal sparkles after animating in
      sparkles.push(makeSparkle({cx, cy, radiusX, radiusY, endAngle: Math.random() * Math.PI + Math.PI, lineWidth, color: boolRandom() ? '#fff' : colorSparkle}))
    }
  }
  
  // PAINT THE SPARKLES
  // and get next sparkles ready too
  const nextSparkles = []
  for (let i = 0, len = sparkles.length; i < len; i++) {
    const {x, y, opacity, color, rad} = sparkles[i]
    ctx.beginPath()
    ctx.globalAlpha = opacity;
    ctx.fillStyle = color
    ctx.arc(x - rad, y - rad, rad, 0, Math.PI / 2);
    ctx.arc(x - rad, y + rad, rad, 3 * Math.PI / 2, 2 * Math.PI);
    ctx.arc(x + rad, y + rad, rad, Math.PI, 3 * Math.PI / 2);
    ctx.arc(x + rad, y - rad, rad, Math.PI / 2, Math.PI);
    ctx.fill()
    
    // Sparkles we are keeping
    if (opacity > .2 && rad > .2) {
      nextSparkles.push({
        x,
        y,
        opacity: opacity - .03,
        rad: rad - .2,
        color
      })
    }
  }
  sparkles = nextSparkles
  
  if (!doneAnimatingIn) {
    // Animating in our rainbow
    requestAnimationFrame(function () {
      animate(percent + inc)
    })
  } else {
    // Resume sparkles on static rainbow
    requestAnimationFrame(function () {
      animate(finish + colors.length * arcStagger)
    })
  }
}

sizeCanvas()
requestAnimationFrame(function () {animate()})
window.addEventListener('resize', sizeCanvas)
              
            
!
999px

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