<main>
  <svg viewBox="0 0 700 700">
    <defs>
      <g id="circles">
        <symbol id="dot">
          <circle cx="30" cy="30" r="30" />
        </symbol>

        <!--
          We need two moving circles in order to pull off this effect:
          one for the gooey effect and another for the glow. By
          animating this circle and using it in the two aforementioned
          scenarios, we guarantee that the two dots we end up using
          will remain moving in sync. This would not be the case if
          we assigned our animation to the two dots separately. Note
          the coordinates that we assign to the circle.
        -->
        <symbol id="moving-dot">
          <circle class="scanner" cx="175" cy="175" r="28" />
        </symbol>
      </g>

      <g id="filters">
        <!-- Our gooey effect filter. -->
        <filter id="goo">
          <feGaussianBlur in="SourceGraphic" stdDeviation="10" result="blur" />
          <feColorMatrix in="blur"
                         mode="matrix"
                         values="1 0 0 0 0
                                 0 1 0 0 0
                                 0 0 1 0 0
                                 0 0 0 19 -9"
                         result="goo" />
          <feComposite in="SourceGraphic" in2="goo" operator="atop" />
        </filter>

        <!--
          This softens the edges of the moving dot by adding a
          subtle glowing effect. We increase the height and the
          width, and adjust the coordinates to avoid clipping.
          These values are arbitrary, as I didn't want to get
          super math-y about it.
        -->
        <filter id="glow" filterUnits="userSpaceOnUse" x="-20%" y="-20%" height="140%" width="140%">
          <feGaussianBlur in="SourceGraphic" stdDeviation="12" />
        </filter>

        <!--
          A larger, yet more subtle glow effect meant for the
          second moving circle. Again, the dimension and position
          attributes are arbitrary but large enough to prevent
          any clipping.
        -->
        <filter id="outer-glow" filterUnits="userSpaceOnUse" x="-50%" y="-50%" height="220%" width="220%">
          <feGaussianBlur in="SourceGraphic" stdDeviation="30" result="outer-glow"/>
        </filter>
      </g>
    </defs>

    <svg aria-labelledby="title"
         aria-describedby="desc"
         aria-busy="true"
         role="progressbar"
         aria-valuemin="0"
         aria-valuemax="100">
      <title id="title">Circular Loader with Gooey Effect</title>
      <desc id="desc">
        A soft cyan glowing dot moving in a circular path defined by eight light blue circles, creating a
        gooey effect as it passes over them.
      </desc>

      <!--
        For the effect to work, the "#goo" filter should be applied
        to a container, which this <g> is. The moving dot uses the
        "#glow" filter to replace the hard edges of the dot with a
        soft glow effect.

        We derive the location of the eight circles by converting
        polar to Cartesian coordinates using the following
        equations:

        x = r cos ϕ
        y = r sin ϕ

        Here, `r` stands for the radius of the circle defining
        the circular path. This is 150 in our case. `ϕ` is
        multiples of 45° (360 divided by 8).
      -->
      <g class="circles centered">
        <use class="dot" href="#dot" x="106"  y="106"/>
        <use class="dot" href="#dot" x="0"    y="150"/>
        <use class="dot" href="#dot" x="-106" y="106"/>
        <use class="dot" href="#dot" x="-150" y="0"/>
        <use class="dot" href="#dot" x="-106" y="-106"/>
        <use class="dot" href="#dot" x="0"    y="-150"/>
        <use class="dot" href="#dot" x="106"  y="-106"/>
        <use class="dot" href="#dot" x="150"  y="0"/>
        <!-- Position the glowing dot by the topmost center dot. -->
        <use class="dot--light" href="#moving-dot" x="-150" y="-150" />
      </g>

      <!--
        This extra dot represents the larger and softer glow around
        the moving dot. We can't include this in the group above
        because the "#goo" filter renders the "#outer-glow" filter
        useless. It causes clipping, in addition, due to it missing
        the filter attributes from "#outer-glow".

        Note that we need to wrap the dot in a <g> due to having
        to apply a translation twice: one for centering the dot and
        another for the animation. If we were to apply the
        `centered` class to the dot, the value of the translation
        from the animation would override it, messing up the
        effect. Alternatively, we could define another `@keyframes`
        rule but with the appropriate `translateY` value.
      -->
      <g class="centered">
        <use class="dot--glowing" href="#moving-dot" x="-150" y="-150" />
      </g>
    </svg>
  </svg>

  <p class="credits">
    Based on the design by
    <a target="_blank" href="https://dribbble.com/shots/2177533-Spinner-Loader-Gooey-light-Effect">
      <b>Christophe Kerebel</b>
    </a>
  </p>
</main>

:root {
  --dot-radius:  30px;
  --path-radius: 150px;
  --vp-width:    700px;
  --vp-height:   700px;

  --static-dot-color: hsl(204, 100%, 65%);
  --moving-dot-color: hsl(178, 94%, 65%);
}


/* --------------------------------------------------
   Elements
   -------------------------------------------------- */

html,
body,
main {
  height: 100vh;
  width: 100vw;
  margin: 0;
}

body {
  background-color: hsl(204, 100%, 9%);
  font-family: sans-serif;
}

main {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
}

p {
  color: var(--static-dot-color);

  font-size: 0.7rem;
  letter-spacing: 0.1rem;
}

a {
  color: var(--moving-dot-color);
  text-decoration: none;
}

/* --------------------------------------------------
   Classes
   -------------------------------------------------- */

.dot {
  fill: var(--static-dot-color);
}

.dot--light {
  fill: var(--moving-dot-color);
  filter: url('#glow');
}

.dot--glowing {
  fill: var(--moving-dot-color);
  filter: url('#outer-glow');
}

.scanner {
  will-change: transform;
  animation: 3s scan 0s infinite linear both;
}

.circles {
  filter: url("#goo");
}

.centered {
  /* FF lets us use percentages, but Chrome doesn't like that. That
   * means we have to hard-code the dimensions of the viewport.
   */
  transform:
    translate(
      calc((var(--vp-width) * 0.5) - var(--dot-radius)),
      calc((var(--vp-height) * 0.5) - var(--dot-radius))
    );
}

.credits {
  margin-bottom: calc(1.5rem * 2);
}


/* --------------------------------------------------
   Animations
   -------------------------------------------------- */

 /* Defines the circular path our dots will take. We use 2 `rotate`
  * functions to contrain the plane containing our dots. Without
  * this, the dots will go out of orbit. See this brilliant post for
  * a much better explanation: 
  * http://www.useragentman.com/blog/2013/03/03/animating-circular-paths-using-css3-transitions/
  */
@keyframes scan {
  from {
    transform: rotate(0deg) translateY(calc(var(--path-radius) * -1)) rotate(0deg);
  }

  to {
    transform: rotate(360deg) translateY(calc(var(--path-radius) * -1)) rotate(-360deg);
  }
}