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CSS transition background color without a hover

I have a div called time-box . Sometimes I will also include an additional class called countdown . If countdown is added then I would like to use a CSS transition effect so the background changes to be red over the course of 60 seconds. In other words, each second that passes the red background gets a little wider until eventually all of the green background has gone.

I have found similar posts here but they all seem to relate to hover

Here is a fiddle

https://jsfiddle.net/e2vbheew/

I don't know a "simple" way to get what you want going from left to right, but there's a way you can create it using before and after pseudoelements. The key here is I'm going to create a :before pseudoelement that has the new background that transitions across, and an :after pseudoelement that replicates the content and puts it on top of the before, so it's still visible. This required putting the content in an attribute on the div so I could reference it in the 'content' of the pseudoelement. If you had more complex content inside, you could probably do away with the :after and simply give the internal content position and z-index to make sure it's visible. Here's the resulting CSS

.time-box {
  height: 27px;                                 
  text-align: center;
  background-color: #25E57B;    
  font-size:2rem;
  padding:0px;
  font-size:1.2rem;
  text-transform:uppercase;
  padding:3px 5px 3px 5px;;
  font-weight:600;
  height:auto;
  position: relative;
}
.time-box:before {
  background-color: red;
  position: absolute;
  left:0;
  top: 0;
  height: 100%;
  width: 0;
  content: " ";
  transition: width 60s ease;
}
.countdown:after {
  content: attr(data-content);
  width: 100%;
  text-align: center;
  height: 100%;
  position: absolute;
  left: 0;
  top: center;
  z-index: 1;
}
.countdown:before {
    width:100%;
}

And updated fiddle: https://jsfiddle.net/tunzwqd7/2/

You would need to add a maximum and a little more math to make the 100% divisible by 60, but this should get you on the right track. Currently this code updates every second and adds 1% to the progress bar width with each iteration.

 var time = 0; var bar = document.querySelector('.countdown .progress-bar'); window.setInterval(function(){ time++; bar.style.width = time+"%"; }, 1000); 
 .time-box { height: 27px; text-align: center; background-color: #25E57B; font-size:2rem; padding:0px; font-size:1.2rem; text-transform:uppercase; padding:3px 5px 3px 5px;; font-weight:600; height:auto; position: relative; } .progress-bar { display: none; } .countdown .progress-bar { display: block; position: absolute; left: 0; top: 0; bottom: 0; background: red; width: 0%; z-index: 1; transition: all 0.3s ease-out; } .countdown p { z-index: 2; position: relative; } 
 <div class="time-box"> <p>12:00</p> <div class="progress-bar"></div> </div> <div class="time-box countdown"> <p>12:00</p> <div class="progress-bar"></div> </div> 

Using CSS animation property...

 .time-box { height: 27px; text-align: center; background-color: #25E57B; font-size: 2rem; padding: 0px; font-size: 1.2rem; text-transform: uppercase; padding: 3px 5px 3px 5px; font-weight: 600; height: auto; position: relative; z-index: 1; } .time-box.countdown:before { content: ''; width: 0; height: 100%; display: block; position: absolute; top: 0; left: 0; background: red; animation: countdown 60s forwards; z-index: -1; } @keyframes countdown { 0% { width: 0; } 100% { width: 100%; } } 
 <div class="time-box"> 12:00 </div> <div class="time-box countdown"> <span>12:00</span> </div> 

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