CodePen Embed - CSS-only chronometer

<div id="wrapper">
  <input type="checkbox" id="button1" checked>
  <label for="button1" data-title="PAUSE">PLAY</label>
  <input type="checkbox" id="button2">
  <label for="button2" data-title="PLAY">STOP</label>
  <span id="fakeStop">STOP</span>
  <div class="digit dp100000 d6">
    <div class="particle top"></div>
    <div class="particle top right"></div>
    <div class="particle top left"></div>
    <div class="particle middle"></div>
    <div class="particle bottom right"></div>
    <div class="particle bottom left"></div>
    <div class="particle bottom"></div>
  </div>
  <div class="digit dp10000">
    <div class="particle top"></div>
    <div class="particle top right"></div>
    <div class="particle top left"></div>
    <div class="particle middle"></div>
    <div class="particle bottom right"></div>
    <div class="particle bottom left"></div>
    <div class="particle bottom"></div>
  </div>
  <div class="separator">
    <div class="dot top"></div>
    <div class="dot bottom"></div>
  </div>
  <div class="digit dp1000 d6">
    <div class="particle top"></div>
    <div class="particle top right"></div>
    <div class="particle top left"></div>
    <div class="particle middle"></div>
    <div class="particle bottom right"></div>
    <div class="particle bottom left"></div>
    <div class="particle bottom"></div>
  </div>
  <div class="digit dp100">
    <div class="particle top"></div>
    <div class="particle top right"></div>
    <div class="particle top left"></div>
    <div class="particle middle"></div>
    <div class="particle bottom right"></div>
    <div class="particle bottom left"></div>
    <div class="particle bottom"></div>
  </div>
  <div class="separator">
    <div class="dot top"></div>
    <div class="dot bottom"></div>
  </div>
  <div class="digit dp10">
    <div class="particle top"></div>
    <div class="particle top right"></div>
    <div class="particle top left"></div>
    <div class="particle middle"></div>
    <div class="particle bottom right"></div>
    <div class="particle bottom left"></div>
    <div class="particle bottom"></div>
  </div>
  <div class="digit dp1">
    <div class="particle top"></div>
    <div class="particle top right"></div>
    <div class="particle top left"></div>
    <div class="particle middle"></div>
    <div class="particle bottom right"></div>
    <div class="particle bottom left"></div>
    <div class="particle bottom"></div>
  </div>
</div>

/* layout */
body {
  background:#333;
  font-family:sans-serif;
}
#wrapper {
  width:260px;
	margin:100px auto;
}

/* general item styles */
.digit {
	position:relative;
	float:left;
	width:30px;
	height:40px;
	box-shadow:0 0 4px rgba(0,0,0,.5);
	margin:0 4px;
}
.particle {
	position:absolute;
	background:#cc0;
	box-shadow:0 0 5px #cc0;
}
.separator {
	position:relative;
	float:left;
	width:8px;
	margin:0 4px;
}
.dot {
	width:8px;
	height:8px;
	background:#cc0;
	box-shadow:0 0 5px #cc0;
	border-radius:4px;
}
input {
	display:none;
}
label, label:before, #fakeStop {
	display:block;
	position:absolute;
	margin:3px 0 0 -50px;
	padding:1px 3px;
	background:#cc0;
	border-radius:2px;
	width:32px;
	font-size:9px;
	cursor:pointer;
}
#fakeStop {
	cursor:default;
	opacity:.4;
}
label:before {
	content:attr(data-title);
	margin:-1px 0 0 -3px;
}
label[for=button2], #fakeStop {
	margin-top:24px;
}
label[for=button2], label:before,
#button1:checked ~ #fakeStop {
	display:none;
}
#button2:checked ~ label[for=button2] {
	margin-top:3px;
}
#button1:checked ~ label[for=button1]:before,
#button1:checked ~ label[for=button2],
#button2:checked ~ label[for=button2]:before,
#button1:checked ~ #button2:checked ~ #fakeStop {
	display:block;
}

/* positioning & dimensions */
.top {
	top:0;
}
.bottom {
	bottom:0;
}
.left {
	left:0;
}
.right {
	right:0;
}
.middle {
	top:17px;
}
.particle.top, .particle.middle, .particle.bottom {
	width:14px;
	height:5px;
	margin:0 0 0 8px;
}
.particle.middle {
	height:6px;
}
.particle.right, .particle.left {
	width:5px;
	height:8px;
	margin:7px 0 0 0;
}
.particle.bottom.right, .particle.bottom.left {
	margin:0 0 7px 0;
}
.dot.top {
	margin-top:8px;
}
.dot.bottom {
	margin-top:8px;
}

/* corners */
.particle:before, .particle:after {
	content:'';
	position:absolute;
	z-index:2;
	display:block;
	width: 0px;
	height: 0px;
	border-style: solid;
}
.particle.top:before {
	margin:0 0 0 -5px;
	border-width: 0 5px 5px 0;
	border-color: transparent #cc0 transparent transparent;
}
.particle.top:after {
	margin:0 0 0 14px;
	border-width: 5px 5px 0 0;
	border-color: #cc0 transparent transparent transparent;
}
.particle.top.right:before {
	margin:-5px 0 0 0;
	border-width: 0 0 5px 5px;
	border-color: transparent transparent #cc0 transparent;
}
.particle.top.right:after {
	margin:8px 0 0 0;
	border-width: 0 5px 3px 0;
	border-color: transparent #cc0 transparent transparent;
}
.particle.top.left:before {
	margin:-5px 0 0 0;
	border-width: 5px 0 0 5px;
	border-color: transparent transparent transparent #cc0;
}
.particle.top.left:after {
	margin:8px 0 0 0;
	border-width: 3px 5px 0 0;
	border-color: #cc0 transparent transparent transparent;
}
.particle.middle:before {
	margin:0 0 0 -5px;
	border-width: 3px 5px 3px 0;
	border-color: transparent #cc0 transparent transparent;
}
.particle.middle:after {
	margin:0 0 0 14px;
	border-width: 3px 0 3px 5px;
	border-color: transparent transparent transparent #cc0;
}
.particle.bottom.right:before {
	margin:-3px 0 0 0;
	border-width: 0 0 3px 5px;
	border-color: transparent transparent #cc0 transparent;
}
.particle.bottom.right:after {
	margin:8px 0 0 0;
	border-width: 0 5px 5px 0;
	border-color: transparent #cc0 transparent transparent;
}
.particle.bottom.left:before {
	margin:-3px 0 0 0;
	border-width: 3px 0 0 5px;
	border-color: transparent transparent transparent #cc0;
}
.particle.bottom.left:after {
	margin:8px 0 0 0;
	border-width: 5px 5px 0 0;
	border-color: #cc0 transparent transparent transparent;
}
.particle.bottom:before {
	margin:0 0 0 -5px;
	border-width: 0 0 5px 5px;
	border-color: transparent transparent #cc0 transparent;
}
.particle.bottom:after {
	margin:0 0 0 14px;
	border-width: 5px 0 0 5px;
	border-color: transparent transparent transparent #cc0;
}

/* animations */
@keyframes initial-visible {
	0% {opacity:1;}
	100% {opacity:1;}
}

@keyframes initial-hidden {
	0% {opacity:.1;}
	100% {opacity:.1;}
}

@keyframes flash {
	0% {opacity:1;}
	49.999% {opacity:1;}
	50% {opacity:.1;}
	100% {opacity:.1;}
}

@keyframes top-10 {
	0% {opacity:1;}
	9.999% {opacity:1;}
	10% {opacity:.1;}
	19.999% {opacity:.1;}
	20% {opacity:1;}
	30% {opacity:1;}
	39.999% {opacity:1;}
	40% {opacity:.1;}
	49.999% {opacity:.1;}
	50% {opacity:1;}
	60% {opacity:1;}
	70% {opacity:1;}
	80% {opacity:1;}
	90% {opacity:1;}
	100% {opacity:1;}
}
@keyframes top-right-10 {
	0% {opacity:1;}
	10% {opacity:1;}
	20% {opacity:1;}
	30% {opacity:1;}
	40% {opacity:1;}
	49.999% {opacity:1;}
	50% {opacity:.1;}
	60% {opacity:.1;}
	69.999% {opacity:.1;}
	70% {opacity:1;}
	80% {opacity:1;}
	90% {opacity:1;}
	100% {opacity:1;}
}
@keyframes top-left-10 {
	0% {opacity:1;}
	9.999% {opacity:1;}
	10% {opacity:.1;}
	20% {opacity:.1;}
	30% {opacity:.1;}
	39.999% {opacity:.1;}
	40% {opacity:1;}
	50% {opacity:1;}
	60% {opacity:1;}
	69.999% {opacity:1;}
	70% {opacity:.1;}
	79.999% {opacity:.1;}
	80% {opacity:1;}
	90% {opacity:1;}
	100% {opacity:1;}
}
@keyframes middle-10 {
	0% {opacity:.1;}
	10% {opacity:.1;}
	19.999% {opacity:.1;}
	20% {opacity:1;}
	30% {opacity:1;}
	40% {opacity:1;}
	50% {opacity:1;}
	60% {opacity:1;}
	69.999% {opacity:1;}
	70% {opacity:.1;}
	79.999% {opacity:.1;}
	80% {opacity:1;}
	90% {opacity:1;}
	100% {opacity:1;}
}
@keyframes bottom-right-10 {
	0% {opacity:1;}
	10% {opacity:1;}
	19.999% {opacity:1;}
	20% {opacity:.1;}
	29.999% {opacity:.1;}
	30% {opacity:1;}
	40% {opacity:1;}
	50% {opacity:1;}
	60% {opacity:1;}
	70% {opacity:1;}
	80% {opacity:1;}
	90% {opacity:1;}
	100% {opacity:1;}
}
@keyframes bottom-left-10 {
	0% {opacity:1;}
	9.999% {opacity:1;}
	10% {opacity:.1;}
	19.999% {opacity:.1;}
	20% {opacity:1;}
	29.999% {opacity:1;}
	30% {opacity:.1;}
	40% {opacity:.1;}
	50% {opacity:.1;}
	59.999% {opacity:.1;}
	60% {opacity:1;}
	69.999% {opacity:1;}
	70% {opacity:.1;}
	79.999% {opacity:.1;}
	80% {opacity:1;}
	89.999% {opacity:1;}
	90% {opacity:.1;}
	100% {opacity:.1;}
}
@keyframes bottom-10 {
	0% {opacity:1;}
	9.999% {opacity:1;}
	10% {opacity:.1;}
	19.999% {opacity:.1;}
	20% {opacity:1;}
	30% {opacity:1;}
	39.999% {opacity:1;}
	40% {opacity:.1;}
	49.999% {opacity:.1;}
	50% {opacity:1;}
	60% {opacity:1;}
	69.999% {opacity:1;}
	70% {opacity:.1;}
	79.999% {opacity:.1;}
	80% {opacity:1;}
	90% {opacity:1;}
	100% {opacity:1;}
}
@keyframes top-6 {
	0% {opacity:1;}
	16.665% {opacity:1;}
	16.666% {opacity:.1;}
	33.332% {opacity:.1;}
	33.333% {opacity:1;}
	50% {opacity:1;}
	66.665% {opacity:1;}
	66.666% {opacity:.1;}
	83.332% {opacity:.1;}
	83.333% {opacity:1;}
	100% {opacity:1;}
}
@keyframes top-right-6 {
	0% {opacity:1;}
	16.666% {opacity:1;}
	33.333% {opacity:1;}
	50% {opacity:1;}
	66.666% {opacity:1;}
	83.332% {opacity:1;}
	83.333% {opacity:.1;}
	100% {opacity:.1;}
}
@keyframes top-left-6 {
	0% {opacity:1;}
	16.665% {opacity:1;}
	16.666% {opacity:.1;}
	33.333% {opacity:.1;}
	50% {opacity:.1;}
	66.665% {opacity:.1;}
	66.666% {opacity:1;}
	83.333% {opacity:1;}
	100% {opacity:1;}
}
@keyframes middle-6 {
	0% {opacity:.1;}
	16.666% {opacity:.1;}
	33.332% {opacity:.1;}
	33.333% {opacity:1;}
	50% {opacity:1;}
	66.666% {opacity:1;}
	83.333% {opacity:1;}
	100% {opacity:1;}
}
@keyframes bottom-right-6 {
	0% {opacity:1;}
	16.666% {opacity:1;}
	33.332% {opacity:1;}
	33.333% {opacity:.1;}
	49.999% {opacity:.1;}
	50% {opacity:1;}
	66.666% {opacity:1;}
	83.333% {opacity:1;}
	100% {opacity:1;}
}
@keyframes bottom-left-6 {
	0% {opacity:1;}
	16.665% {opacity:1;}
	16.666% {opacity:.1;}
	33.332% {opacity:.1;}
	33.333% {opacity:1;}
	49.999% {opacity:1;}
	50% {opacity:.1;}
	66.666% {opacity:.1;}
	83.333% {opacity:.1;}
	100% {opacity:.1;}
}
@keyframes bottom-6 {
	0% {opacity:1;}
	16.665% {opacity:1;}
	16.666% {opacity:.1;}
	33.332% {opacity:.1;}
	33.333% {opacity:1;}
	50% {opacity:1;}
	66.665% {opacity:1;}
	66.666% {opacity:.1;}
	83.332% {opacity:.1;}
	83.333% {opacity:1;}
	100% {opacity:1;}
}

.particle, .separator {
	animation-iteration-count: infinite;
	animation-timing-function: linear;
	animation-play-state: paused;
}
.particle.top { animation-name: top-10; }
.particle.top.right { animation-name: top-right-10; }
.particle.top.left { animation-name: top-left-10; }
.particle.middle { animation-name: middle-10; }
.particle.bottom.right { animation-name: bottom-right-10; }
.particle.bottom.left { animation-name: bottom-left-10; }
.particle.bottom { animation-name: bottom-10; }

.d6 .particle.top { animation-name: top-6; }
.d6 .particle.top.right { animation-name: top-right-6; }
.d6 .particle.top.left { animation-name: top-left-6; }
.d6 .particle.middle { animation-name: middle-6; }
.d6 .particle.bottom.right { animation-name: bottom-right-6; }
.d6 .particle.bottom.left { animation-name: bottom-left-6; }
.d6 .particle.bottom { animation-name: bottom-6; }

.dp1 .particle { animation-duration: .1s; }
.dp10 .particle { animation-duration: 1s; }
.dp100 .particle { animation-duration: 10s; }
.dp1000 .particle { animation-duration: 60s; }
.dp10000 .particle { animation-duration: 600s; }
.dp100000 .particle { animation-duration: 3600s; }

.separator {
	animation-name: flash;
	animation-duration: 1s;
}

#button1:checked ~ .digit .particle,
#button1:checked ~ .separator {
	animation-play-state: running;
}
#button2:checked ~ .digit .particle,
#button2:checked ~ .separator {
	animation-name: initial-visible;
}
#button2:checked ~ .digit .particle.middle {
	animation-name: initial-hidden;
}

This Pen is owned by kachibito on CodePen. It is a fork of this Pen.

See more by @kachibito on CodePen

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js