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HTML 

              
                Building a Vertical Timeline With CSS and a Touch of JavaScript
original by <a href="https://webdesign.tutsplus.com/tutorials/building-a-vertical-timeline-with-css-and-a-touch-of-javascript--cms-26528"> George Martsoukos on 24 May 2016</a>
Original codepen
<a href="https://codepen.io/tutsplus/details/QNeJgR"> is here </a>
<br>
scroll and resize to view effetcs
<div class="timeline2">
		  <ul>
			<li>
			  <div>
				<time>1934</time> At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium
			  </div>
				<span class="time">
					<span>JUN, 17<sup>th</sup></span>
					<span>12:00 AM</span>
				</span>
			</li>
			<li>
			  <div>
				<time>1937</time> Proin quam velit, efficitur vel neque vitae, rhoncus commodo mi. Suspendisse finibus mauris et bibendum molestie. Aenean ex augue, varius et pulvinar in, pretium non nisi.
			  </div>
				<span class="time">
					<span>JUN, 29<sup>th</sup></span>
					<span>11:36 AM</span>
				</span>
			</li>
			<li>
			  <div>
				<time>1940</time> Proin iaculis, nibh eget efficitur varius, libero tellus porta dolor, at pulvinar tortor ex eget ligula. Integer eu dapibus arcu, sit amet sollicitudin eros.
			  </div>
				<span class="time">
					<span>Jul, 1<sup>st</sup></span>
					<span>10:30 AM</span>
				</span>
			</li>
			<li>
			  <div>
				<time>1943</time> In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
			  </div>
				<span class="time">
					<span>Jul, 2<sup>nd</sup></span>
					<span>12:30 PM</span>
				</span>
			</li>
			<li>
			  <div>
				<time>1946</time> In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
			  </div>
				<span class="time">
					<span>Jul, 3<sup>rd</sup></span>
					<span>01:30 PM</span>
				</span>
			</li>
			<li>
			  <div>
				<time>1956</time> In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
			  </div>
				<span class="time">
					<span>Jul, 4<sup>th</sup></span>
					<span>01:30 PM</span>
				</span>
			</li>
			<li>
			  <div>
				<time>1957</time> In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
			  </div>
				<span class="time">
					<span>Jul, 5<sup>th</sup></span>
					<span>01:30 PM</span>
				</span>
			</li>
			<li>
			  <div>
				<time>1967</time> Aenean condimentum odio a bibendum rhoncus. Ut mauris felis, volutpat eget porta faucibus, euismod quis ante.
			  </div>
				<span class="time">
					<span>Jul, 6<sup>th</sup></span>
					<span>01:30 PM</span>
				</span>
			</li>
			<li>
			  <div>
				<time>1977</time> Vestibulum porttitor lorem sed pharetra dignissim. Nulla maximus, dui a tristique iaculis, quam dolor convallis enim, non dignissim ligula ipsum a turpis.
			  </div>
				<span class="time">
					<span>Jul, 7<sup>th</sup></span>
					<span>01:30 PM</span>
				</span>
			</li>
			<li>
			  <div>
				<time>1985</time> In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
			  </div>
				<span class="time">
					<span>Aug, 1<sup>st</sup></span>
					<span>01:30 PM</span>
				</span>
			</li>
			<li>
			  <div>
				<time>2000</time> In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
			  </div>
				<span class="time">
					<span>Aug, 2<sup>nd</sup></span>
					<span>01:30 PM</span>
				</span>
			</li>
			<li>
			  <div>
				<time>2005</time> In mattis elit vitae odio posuere, nec maximus massa varius. Suspendisse varius volutpat mattis. Vestibulum id magna est.
			  </div>
				<span class="time">
					<span>Aug, 3<sup>rd</sup></span>
					<span>01:30 PM</span>
				</span>
			</li>
		  </ul>
		</div>
!

CSS 

              
                /* TIMELINE
			–––––––––––––––––––––––––––––––––––––––––––––––––– */
			.timeline2 {
				overflow-y: scroll;
				overflow-x: hidden;
				height: 550px;
				top: calc(100%-600px);
				position: relative;
				scroll-behavior:  smooth ;
				 
			}
			.timeline2   *, .timeline2   *::before, .timeline2   *::after { 
				font: normal 16px/1.5 "Helvetica Neue", sans-serif;
				color: #fff;
				margin: 0;
				padding: 0;
				box-sizing: border-box;
			}
			.timeline2::-webkit-scrollbar {
				width: 12px;
				background-color: #F5F5F5;
			}
			 
			.timeline2::-webkit-scrollbar-track {
				-webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.3);
				border-radius: 10px;
				background-color: #F5F5F5;
			}
			 
			.timeline2::-webkit-scrollbar-thumb {
				border-radius: 10px;
				-webkit-box-shadow: inset 0 0 6px rgba(0,0,0,.3);
				background-color: #555;
			}
			 
			.timeline2 ul {
			  background: #456990;
			  /* padding: 50px 0; */
			}

			.timeline2 ul li {
			  list-style-type: none;
			  position: relative;
			      width: 2px;/* width: 6px; */
			  margin: 0 auto;
				padding-top: 10px;/* 50px; */
				padding-bottom: 10px;/* padding-bottom: 20px; = center padding-bottom: 10px; = center */
			 background: #fff; /* background: black;  */
			  
				border-left: 3px double white;
				
			}

			 
			.timeline2 ul li::after {
			  content: '';
			  position: absolute;
			  left: 50%;
			  top: 30px; /* bottom: 0;  bottom: 50%;=center top: 30px;=top  */ 
			  transform: translateX(-55%);/* transform: translateX(-50%); */
			  -webkit-transform: translateX(-55%);
			  width: 30px;
			  height: 30px;
			  border-radius: 50%;
			  background: inherit;
			  border: 2px solid #03A9F4;
			}

			.timeline2 ul li div {
			  position: relative;
			  bottom: 0;
			  width: 400px;
			  padding: 15px;
			  background: #03A9F4;
			  
			}

			.timeline2 ul li div::before {
			  content: '';
			  position: absolute;
			  top: 20px; /* bottom: 7px; bottom: 50%;=center top: 20px;=top */
			  
			  width: 0;
			  height: 0;
			  border-style: solid;
			}

			.timeline2 ul li:nth-child(odd) div {
			  left: 45px;
			}

			.timeline2 ul li:nth-child(odd) div::before {
			  left: -15px;
				border-width: 16px 16px 16px 0;/* border-width: 8px 16px 8px 0; */
			  border-color: transparent #03A9F4 transparent transparent;
			}

			.timeline2 ul li:nth-child(even) div {
			  left: -439px;
			}

			.timeline2 ul li:nth-child(even) div::before {
			  right: -15px;
			  	border-width: 16px 0 16px 16px; /* border-width: 8px 0 8px 16px; */
			  border-color: transparent transparent transparent #03A9F4;
			}
			
			
			.timeline2 ul li:nth-child(odd) .time {
				position: absolute;
				top: 20px;
				left: -100px;
				width: 90px;
			}
			.timeline2 ul li:nth-child(even) .time {
				position: absolute;
				top: 20px;
				right: -120px;
				width: 90px;
			}
			
			
			.timeline2 time {
			  display: block;
			  font-size: 1.2rem;
			  font-weight: bold;
			  margin-bottom: 8px;
			}


			/* EFFECTS
			–––––––––––––––––––––––––––––––––––––––––––––––––– */

			.timeline2 ul li::after {
				transition: background .5s ease-in-out;
				-webkit-transition: background .5s ease-in-out;
			}

			.timeline2 ul li.in-view::after {
			  background: #03A9F4;
			  border: 2px solid white;
			}

			.timeline2 ul li div {
			  visibility: hidden;
			  opacity: 0;
				transition: all .5s ease-in-out;
				-webkit-transition: all .5s ease-in-out;
			}

			.timeline2 ul li:nth-child(odd) div {
				transform: translate3d(200px, 0, 0);
				-webkit-transform: translate(200px, 0, 0);
			}

			.timeline2 ul li:nth-child(even) div {
				transform: translate3d(-200px, 0, 0);
				-webkit-transform: translate(-200px, 0, 0);
			}

			.timeline2 ul li.in-view div {
				transform: none;
				-webkit-transform: none;
			  visibility: visible;
			  opacity: 1;
			  /* disabled when arrow is on top position
			  border-radius: 10px;
			  */
			}
			
			.timeline2 ul li  .time {
				transition: all .5s ease-in-out;
				-webkit-transition: all .5s ease-in-out;
				transform: rotate(-90deg);
				-webkit-transform:rotate(270deg);
			}
			.timeline2 ul li.in-view  .time {
				transform: rotate(0deg);
				-webkit-transform:rotate(0deg);
			}

			/* GENERAL MEDIA QUERIES
			–––––––––––––––––––––––––––––––––––––––––––––––––– */

			@media screen and (max-width: 900px) {
			  .timeline2 ul li div {
				width: 250px;
			  }
			  .timeline2 ul li:nth-child(even) div {
				left: -289px;
				/*250+45-6*/
			  }
			}

			@media screen and (max-width: 600px) {
			  .timeline2 ul li {
				margin-left: 20px;
			  }
			  .timeline2 ul li div {
				width: calc(100vw - 120px);
			  }
			  .timeline2 ul li:nth-child(even) div {
				left: 45px;
			  }
			  .timeline2 ul li:nth-child(even) div::before {
				left: -15px;
				border-width: 16px 16px 16px 0; /* border-width: 8px 16px 8px 0;  */
				border-color: transparent #03A9F4 transparent transparent;
			  }
			  
				
				.timeline2 ul li:nth-child(odd) .time {
					position: absolute;
					top: 60px;
					left: -23px;
					width: 90px;
					/*
					transform: rotate(-90deg);
					-webkit-transform:rotate(270deg);
					*/
					transform: rotate(0deg);
					-webkit-transform:rotate(0deg);
				}
				.timeline2 ul li:nth-child(even) .time {
					position: absolute;
					top: 60px;
					right: -67px;
					width: 90px;
					/*
					transform: rotate(-90deg);
					-webkit-transform:rotate(270deg);
					*/
					transform: rotate(0deg);
					-webkit-transform:rotate(0deg);
				}
				.timeline2 ul li.in-view:nth-child(odd) .time {
					transform: rotate(-90deg);
					-webkit-transform:rotate(270deg);					 
				}
				.timeline2 ul li.in-view:nth-child(even) .time {
					transform: rotate(-90deg);
					-webkit-transform:rotate(270deg);
				}
			  
			}
!

JS 

              
                (function() {
			 
			  'use strict';

			  // define variables
			  var timelines= document.querySelectorAll('.timeline2');
			   
				function debounce(func, wait, immediate) {
					var timeout;
					return function() {
						var context = this, args = arguments;
						var later = function() {
							timeout = null;
							if (!immediate) func.apply(context, args);
						};
						var callNow = immediate && !timeout;
						clearTimeout(timeout);
						timeout = setTimeout(later, wait);
						if (callNow) func.apply(context, args);
					};
				}
				function callbackFunc() {
					  var h,timeline, li,rect,parent_rect,i,items;
					for(h=0;h<timelines.length;h++){
						  timeline=timelines[h];
							parent_rect=timeline.getBoundingClientRect();
						   items = timeline.querySelectorAll(".timeline2 li");
						for (  i = 0; i < items.length; i++) {
							/*
						  if (isElementInViewport(items[i])) {
							items[i].classList.add("in-view");				   
						  }
						  */
							li=items[i];
							rect = li.getBoundingClientRect();  
							 
							if( (rect.bottom<=(parent_rect.top+(rect.height/2) ) ) || (rect.top >=(parent_rect.bottom-(rect.height/2)) ) ){
								//debugger;
								//li.style['background']='red';
								li.classList.remove("in-view");
								
							}else{
								//li.style['background']='white';
								li.classList.add("in-view");
							}
							 
						}
					}
				}
				var updateLayout =debounce(function(e) {

					// Does all the layout updating here
					callbackFunc();
					
				}, 500); // Maximum run of once per 500 milliseconds

				// listen for events
				window.addEventListener("load", callbackFunc);
				window.addEventListener("resize", updateLayout);
				window.addEventListener("scroll", callbackFunc);
				for(var h=0;h<timelines.length;h++){
						var  timeline=timelines[h];
					timeline.addEventListener("scroll",callbackFunc );
				}
				
			 })();
!
999px

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