Pure CSS countdown animation

Question

I'm trying to do a pure HTML/CSS countdown animation but I'm struggling to make it work smoothly. For now I'm only covering minutes and seconds. Notice that I'm using animation-delay property for resetting my countdown to a certain initial time.

For now everything is working fine, except the digit corresponding to the x10 minutes which is not changing sync with the rest of the digits. Following I will paste my code and two codepens, a first normal version with the correct timings for a normal behaviour and another one 10 times faster so you can quickly appreciate the problem.

• https://codepen.io/ecazorla/pen/WKdZpQ (normal version)
• https://codepen.io/ecazorla/pen/bjapeN (10 times faster version)

@keyframes tick6 {
        0%      { margin-top: 0; }
        16%     { margin-top: -2rem;  }
        33%     { margin-top: -4rem;  }
        50%     { margin-top: -6rem;  }
        66%     { margin-top: -8rem;  }
        83%     { margin-top: -10rem;  }
        100%    { margin-top: -12rem;  }
    }

    @keyframes tick10 {
        0%      { margin-top: 0; }
        10%     { margin-top: -2rem;  }
        20%     { margin-top: -4rem;  }
        30%     { margin-top: -6rem;  }
        40%     { margin-top: -8rem;  }
        50%     { margin-top: -10rem;  }
        60%     { margin-top: -12rem;  }
        70%     { margin-top: -14rem;  }
        80%     { margin-top: -16rem;  }
        90%     { margin-top: -18rem;  }
        100%    { margin-top: -20rem;  }
    }
    
body {
  background-color: black;
}

.container {
  background-color: white;
}

.digit {
  display: inline-block;
  height: 2rem;
  overflow: hidden;
  width: 1ch;
}

.digit span {
  display: block;
  height: 2rem;
  width: 100%;
}

.minutes-digit-one {
  animation: tick6 3600s step-end;
  animation-iteration-count: infinite;
  animation-delay: -540s;
}

.minutes-digit-two {
  animation: tick10 600s step-end;
  animation-iteration-count: infinite;
  animation-delay: -540s;
}

.seconds-digit-one {
  animation: tick6 60s step-end;
  animation-iteration-count: infinite;
  animation-delay: -540s;
}

.seconds-digit-two {
  animation: tick10 10s;
  animation-iteration-count: infinite;
  animation-delay: -540s;
}

<div class="container">
  <div class="digit">
    <span class="minutes-digit-one">5</span>
    <span>4</span>
    <span>3</span>
    <span>2</span>
    <span>1</span>
    <span>0</span>
    <span>5</span>
  </div>
  <div class="digit">
    <span class="minutes-digit-two">9</span>
    <span>8</span>
    <span>7</span>
    <span>6</span>
    <span>5</span>
    <span>4</span>
    <span>3</span>
    <span>2</span>
    <span>1</span>
    <span>0</span>
    <span>9</span>
  </div>
  <div class="digit">
    <span>:</span>
  </div>
  <div class="digit">
    <span class="seconds-digit-one">5</span>
    <span>4</span>
    <span>3</span>
    <span>2</span>
    <span>1</span>
    <span>0</span>
    <span>5</span>
  </div>
  <div class="digit">
    <span class="seconds-digit-two">9</span>
    <span>8</span>
    <span>7</span>
    <span>6</span>
    <span>5</span>
    <span>4</span>
    <span>3</span>
    <span>2</span>
    <span>1</span>
    <span>0</span>
    <span>9</span>
  </div>
</div>

Answer 1

This is due to the percentage values of the keyframes. You are using 16% while it should be 16.6667% (100/6) to be more exact and accurate, same thing with the others values.

When using 360s or 3600s as a duration you will have a clear difference between 16% and 16.6667% thus the state will change earlier and you will have the synchronisation issue. You may not notice this when using 6s as duration with the seconds.

@keyframes tick6 {  
    0%          { margin-top: 0; }      /* 0*(100/6) */
    16.6667%    { margin-top: -2rem;  } /* 1*(100/6) */
    33.3333%    { margin-top: -4rem;  } /* 2*(100/6) */
    50%         { margin-top: -6rem;  } /* 3*(100/6) */
    66.6667%    { margin-top: -8rem;  } /* 4*(100/6) */
    83.3333%    { margin-top: -10rem; } /* 5*(100/6) */
    100%        { margin-top: -12rem; } /* 6*(100/6) */
}

@keyframes tick10 {
    0%      { margin-top: 0; }
    10%     { margin-top: -2rem;  }
    20%     { margin-top: -4rem;  }
    30%     { margin-top: -6rem;  }
    40%     { margin-top: -8rem;  }
    50%     { margin-top: -10rem;  }
    60%     { margin-top: -12rem;  }
    70%     { margin-top: -14rem;  }
    80%     { margin-top: -16rem;  }
    90%     { margin-top: -18rem;  }
    100%    { margin-top: -20rem;  }
}

body {
  background-color: black;
}

.container {
  background-color: white;
}

.digit {
  display: inline-block;
  height: 2rem;
  overflow: hidden;
  width: 1ch;
}

.digit span {
  display: block;
  height: 2rem;
  width: 100%;
}

.minutes-digit-one {
  animation: tick6 360s infinite step-end;
  /*animation-delay: -54s;*/
}

.minutes-digit-two {
  animation: tick10 60s infinite step-end;
  /*animation-delay: -54s;*/
}

.seconds-digit-one {
  animation: tick6 6s infinite step-end;
  /*animation-delay: -54s;*/
}

.seconds-digit-two {
  animation: tick10 1s infinite step-end;
}

<div class="container">
  <div class="digit">
    <span class="minutes-digit-one">5</span>
    <span>4</span>
    <span>3</span>
    <span>2</span>
    <span>1</span>
    <span>0</span>
    <span>5</span>
  </div>
  <div class="digit">
    <span class="minutes-digit-two">9</span>
    <span>8</span>
    <span>7</span>
    <span>6</span>
    <span>5</span>
    <span>4</span>
    <span>3</span>
    <span>2</span>
    <span>1</span>
    <span>0</span>
    <span>9</span>
  </div>
  <div class="digit">
    <span>:</span>
  </div>
  <div class="digit">
    <span class="seconds-digit-one">5</span>
    <span>4</span>
    <span>3</span>
    <span>2</span>
    <span>1</span>
    <span>0</span>
    <span>5</span>
  </div>
  <div class="digit">
    <span class="seconds-digit-two">9</span>
    <span>8</span>
    <span>7</span>
    <span>6</span>
    <span>5</span>
    <span>4</span>
    <span>3</span>
    <span>2</span>
    <span>1</span>
    <span>0</span>
    <span>9</span>
  </div>
</div>