To avoid confusing the user when looking for a page that can not be found, page 404 can be used. In this code, this page has been prepared with a simple but beautiful design. The phrase 404 is in color in the middle of this page. The characters in this phrase are colored one by one. This small change has increased the beauty of this 404 page.


HTML
<!-- This script got from www.devanswer.com -->
<h1>404 Error Page </h1>
<p class="zoom-area"><b>CSS</b> animations to make a cool 404 page. </p>
<section class="error-container">
  <span>4</span>
  <span><span class="screen-reader-text">0</span></span>
  <span>4</span>
</section><div id="bcl"><a style="font-size:8pt;text-decoration:none;" href="http://www.devanswer.com">Developers Answer</a></div>
                        

CSS
@import url('https://fonts.googleapis.com/css?family=Montserrat:400,600,700');
@import url('https://fonts.googleapis.com/css?family=Catamaran:400,800');
.error-container {
  text-align: center;
  font-size: 180px;
  font-family: 'Catamaran', sans-serif;
  font-weight: 800;
  margin: 20px 15px;
}
.error-container > span {
  display: inline-block;
  line-height: 0.7;
  position: relative;
  color: #FFB485;
}
.error-container > span {
  display: inline-block;
  position: relative;
  vertical-align: middle;
}
.error-container > span:nth-of-type(1) {
  color: #D1F2A5;
  animation: colordancing 4s infinite;
}
.error-container > span:nth-of-type(3) {
  color: #F56991;
  animation: colordancing2 4s infinite;
}
.error-container > span:nth-of-type(2) {
  width: 120px;
  height: 120px;
  border-radius: 999px;
}
.error-container > span:nth-of-type(2):before,
.error-container > span:nth-of-type(2):after {
	border-radius: 0%;
	content:"";
	position: absolute;
	top: 0; left: 0;
	width: inherit; height: inherit;
  border-radius: 999px;
	box-shadow: inset 30px 0 0 rgba(209, 242, 165, 0.4),
				inset 0 30px 0 rgba(239, 250, 180, 0.4),
				inset -30px 0 0 rgba(255, 196, 140, 0.4),	
				inset 0 -30px 0 rgba(245, 105, 145, 0.4);
  animation: shadowsdancing 4s infinite;
}
.error-container > span:nth-of-type(2):before {
	-webkit-transform: rotate(45deg);
	   -moz-transform: rotate(45deg);
			transform: rotate(45deg);
}
.screen-reader-text {
    position: absolute;
    top: -9999em;
    left: -9999em;
}
@keyframes shadowsdancing {
  0% {
    box-shadow: inset 30px 0 0 rgba(209, 242, 165, 0.4),
				inset 0 30px 0 rgba(239, 250, 180, 0.4),
				inset -30px 0 0 rgba(255, 196, 140, 0.4),	
				inset 0 -30px 0 rgba(245, 105, 145, 0.4);
  }
  25% {
    box-shadow: inset 30px 0 0 rgba(245, 105, 145, 0.4),
				inset 0 30px 0 rgba(209, 242, 165, 0.4),
				inset -30px 0 0 rgba(239, 250, 180, 0.4),	
				inset 0 -30px 0 rgba(255, 196, 140, 0.4);
  }
  50% {
     box-shadow: inset 30px 0 0 rgba(255, 196, 140, 0.4),
				inset 0 30px 0 rgba(245, 105, 145, 0.4),
				inset -30px 0 0 rgba(209, 242, 165, 0.4),	
				inset 0 -30px 0 rgba(239, 250, 180, 0.4);
  }
  75% {
   box-shadow: inset 30px 0 0 rgba(239, 250, 180, 0.4),
				inset 0 30px 0 rgba(255, 196, 140, 0.4),
				inset -30px 0 0 rgba(245, 105, 145, 0.4),	
				inset 0 -30px 0 rgba(209, 242, 165, 0.4);
  }
  100% {
    box-shadow: inset 30px 0 0 rgba(209, 242, 165, 0.4),
				inset 0 30px 0 rgba(239, 250, 180, 0.4),
				inset -30px 0 0 rgba(255, 196, 140, 0.4),	
				inset 0 -30px 0 rgba(245, 105, 145, 0.4);
  }
}
@keyframes colordancing {
  0% {
    color: #D1F2A5;
  }
  25% {
    color: #F56991;
  }
  50% {
    color: #FFC48C;
  }
  75% {
    color: #EFFAB4;
  }
  100% {
    color: #D1F2A5;
  }
}
@keyframes colordancing2 {
  0% {
    color: #FFC48C;
  }
  25% {
    color: #EFFAB4;
  }
  50% {
    color: #D1F2A5;
  }
  75% {
    color: #F56991;
  }
  100% {
    color: #FFC48C;
  }
}
/* demo stuff */
* {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}
body {
  background-color: #416475;
  margin-bottom: 50px;
}
html, button, input, select, textarea {
    font-family: 'Montserrat', Helvetica, sans-serif;
    color: #92a4ad;
}
h1 {
  text-align: center;
  margin: 30px 15px;
}
.zoom-area { 
  max-width: 490px;
  margin: 30px auto 30px;
  font-size: 19px;
  text-align: center;
}