Как использовать canvas фоном для div

"use strict"; { 
  const perlin = { 
    init() { 
      this.p = new Uint8Array(512); 
      this.reset(); 
    }, 
    reset() { 
      const p = new Uint8Array(256); 
      for (let i = 0; i < 256; i++) p[i] = i; 
      for (let i = 255; i > 0; i--) { 
        const n = Math.floor((i + 1) * Math.random()); 
        [p[i], p[n]] = [p[n], p[i]]; 
      } 
      for (let i = 0; i < 512; i++) this.p[i] = p[i & 255]; 
    }, 
    lerp(t, a, b) { 
      return a + t * (b - a); 
    }, 
    grad2d(i, x, y) { 
      const v = (i & 1) === 0 ? x : y; 
      return (i & 2) === 0 ? -v : v; 
    }, 
    noise2d(x2d, y2d) { 
      const X = Math.floor(x2d) & 255; 
      const Y = Math.floor(y2d) & 255; 
      const x = x2d - Math.floor(x2d); 
      const y = y2d - Math.floor(y2d); 
      const fx = (3 - 2 * x) * x * x; 
      const fy = (3 - 2 * y) * y * y; 
      const p0 = this.p[X] + Y; 
      const p1 = this.p[X + 1] + Y; 
      return this.lerp( 
        fy, 
        this.lerp( 
          fx, 
          this.grad2d(this.p[p0], x, y), 
          this.grad2d(this.p[p1], x - 1, y) 
        ), 
        this.lerp( 
          fx, 
          this.grad2d(this.p[p0 + 1], x, y - 1), 
          this.grad2d(this.p[p1 + 1], x - 1, y - 1) 
        ) 
      ); 
    } 
  }; 
  ///////////////////////////////////////////////////////////////// 
  const canvas = { 
    init() { 
      this.elem = document.getElementById("canvas"); 
      document.body.appendChild(this.elem); 
      this.width = this.elem.width = this.elem.offsetWidth; 
      this.height = this.elem.height = this.elem.offsetHeight; 
      return this.elem.getContext("2d"); 
    } 
  }; 
  ///////////////////////////////////////////////////////////////// 
  const webgl = { 
    init(canvas, options) { 
      this.elem = document.createElement("canvas"); 
      this.gl = ( 
        this.elem.getContext("webgl", options) || 
        this.elem.getContext("experimental-webgl", options) 
      ); 
      if (!this.gl) return false; 
      const vertexShader = this.gl.createShader(this.gl.VERTEX_SHADER); 
      this.gl.shaderSource(vertexShader, ` 
				precision highp float; 
				attribute vec3 aPosition; 
				uniform vec2 uResolution; 
				void main() { 
					gl_PointSize = 1.0; 
					gl_Position = vec4( 
						( aPosition.x / uResolution.x * 2.0) - 1.0,  
						(-aPosition.y / uResolution.y * 2.0) + 1.0,  
						0.0, 
						1.0 
					); 
				}`); 
      this.gl.compileShader(vertexShader); 
      const fragmentShader = this.gl.createShader(this.gl.FRAGMENT_SHADER); 
      this.gl.shaderSource(fragmentShader, ` 
				precision highp float; 
				void main() { 
					gl_FragColor = vec4(0.07, 0.0, 0.68, 1.0); 
				}`); 
      this.gl.compileShader(fragmentShader); 
      const program = this.gl.createProgram(); 
      this.gl.attachShader(program, vertexShader); 
      this.gl.attachShader(program, fragmentShader); 
      this.gl.linkProgram(program); 
      this.gl.useProgram(program); 
      this.aPosition = this.gl.getAttribLocation(program, "aPosition"); 
      this.gl.enableVertexAttribArray(this.aPosition); 
      this.positionBuffer = this.gl.createBuffer(); 
      this.elem.width = canvas.width; 
      this.elem.height = canvas.height; 
      const uResolution = this.gl.getUniformLocation(program, "uResolution"); 
      this.gl.enableVertexAttribArray(uResolution); 
      this.gl.uniform2f(uResolution, canvas.width, canvas.height); 
      this.gl.viewport( 
        0, 
        0, 
        this.gl.drawingBufferWidth, 
        this.gl.drawingBufferHeight 
      ); 
      return this.gl; 
    }, 
    drawBuffer(data, num) { 
      this.gl.bindBuffer(this.gl.ARRAY_BUFFER, this.positionBuffer); 
      this.gl.vertexAttribPointer(this.aPosition, 2, this.gl.FLOAT, false, 0, 0); 
      this.gl.bufferData( 
        this.gl.ARRAY_BUFFER, 
        data, 
        this.gl.DYNAMIC_DRAW 
      ); 
      this.gl.drawArrays(this.gl.GL_POINTS, 0, num); 
    } 
  }; 
  ///////////////////////////////////////////////////////////////// 
  const ctx = canvas.init(); 
  const gl = webgl.init(canvas, { 
    alpha: false, 
    stencil: false, 
    antialias: false, 
    depth: false, 
  }); 
  perlin.init(); 
  const nParticles = 30000; 
  const velocities = new Float32Array(nParticles * 2); 
  const particles = new Float32Array(nParticles * 2); 
  let frame = 0; 
  for (let i = 0; i < nParticles; i++) { 
    const p = i * 2; 
    particles[p + 0] = Math.random() * canvas.width; 
    particles[p + 1] = Math.random() * canvas.height; 
  } 
  ///////////////////////////////////////////////////////////////// 
  const run = () => { 
    requestAnimationFrame(run); 
    frame++; 
    gl.clear(gl.COLOR_BUFFER_BIT); 
    ctx.globalCompositeOperation = "source-over"; 
    ctx.fillStyle = "rgba(0, 0, 0, 0.025)"; 
    ctx.fillRect(0, 0, canvas.width, canvas.height); 
    ctx.globalCompositeOperation = "lighter"; 
    for (let i = 0; i < nParticles; i++) { 
      const p = i * 2; 
      let n = 80 * perlin.noise2d(particles[p + 0] * 0.001, particles[p + 1] * 0.001); 
      velocities[p + 0] += 0.1 * Math.cos(n); 
      velocities[p + 1] += 0.1 * Math.sin(n); 
      particles[p + 0] += (velocities[p + 0] *= 0.99); 
      particles[p + 1] += (velocities[p + 1] *= 0.99); 
      particles[p + 0] = (canvas.width + particles[p + 0]) % canvas.width; 
      particles[p + 1] = (canvas.height + particles[p + 1]) % canvas.height; 
    } 
    webgl.drawBuffer(particles, nParticles); 
    if (frame > 30) ctx.drawImage(webgl.elem, 0, 0); 
  }; 
  requestAnimationFrame(run); 
  ///////////////////////////////////////////////////////////////// 
  ["click", "touchdown"].forEach(event => { 
    document.addEventListener(event, e => perlin.reset(), false); 
  }); 
}

html, body { 
  width: 100%; 
  height: 100%; 
  margin: 0; 
} 
 
body { 
  position: relative; 
  z-index: 0; 
   
  color: white; 
  text-align: center; 
  font-size: 3em; 
} 
 
canvas { 
  position: absolute; 
  left: 0; 
  top: 0; 
  right: 0; 
  bottom: 0; 
  width: 100%; 
  height: 100%; 
  background: #000; 
  z-index: -1; 
}

<canvas id="canvas"></canvas> 
А тут какой-то текст поверх канваса. 
<br> 
<a href=http://www.w3schools.com/html/html5_video.asp>И даже ссылка</a>