Smooth scrolling slider with play/pause button

I’m doing a project where I need a scrollable slider with play pause button like www.gap.com. I got this below code from W3C but not sure why multiple images are not showing fully. If I change the width value in CSS code, only first image portion scrolls but it totally ignores the 2nd image. Please anyone help me.

var speed=20        // speed of scroller
var step=3          // smoothness of movement
var StartActionText= "Scroll"  // Text for start link
var StopActionText = "Pause"   // Text for stop link

var x, scroll, divW, sText=""

function onclickIE(idAttr,handler,call){
  if ((document.all)&&(document.getElementById)){idAttr[handler]="Javascript:"+call}
}

function addLink(id,call,txt){
  var e=document.createElement('a')
  e.setAttribute('href',call)
  var linktext=document.createTextNode(txt)
  e.appendChild(linktext)
  document.getElementById(id).appendChild(e)
}

function getElementStyle() {
    var elem = document.getElementById('scroller');
    if (elem.currentStyle) {
        return elem.currentStyle.overflow;
    } else if (window.getComputedStyle) {
        var compStyle = window.getComputedStyle(elem, '');
        return compStyle.getPropertyValue("overflow");
    }
    return "";
}

function addControls(){

// test for CSS support first 
// test for the overlow property value set in style element or external file

if (getElementStyle()=="hidden") {
  var f=document.createElement('div');
  f.setAttribute('id','controls');
  document.getElementById('scroller').parentNode.appendChild(f);
  addLink('controls','Javascript:clickAction(0)',StopActionText);
  onclickIE(document.getElementById('controls').childNodes[0],"href",'clickAction(0)');
  document.getElementById('controls').style.display='block';
  }
}

function stopScroller(){clearTimeout(scroll)}

function setAction(callvalue,txt){
  var c=document.getElementById('controls')
  c.childNodes[0].setAttribute('href','Javascript:clickAction('+callvalue+')')
  onclickIE(document.getElementById('controls').childNodes[0],"href",'clickAction('+callvalue+')')
  c.childNodes[0].firstChild.nodeValue=txt
}

function clickAction(no){
  switch(no) {
    case 0:
      stopScroller();
      setAction(1,StartActionText);
      break;
    case 1:
      startScroller();
      setAction(0,StopActionText);
  }
}



function startScroller(){
  document.getElementById('tag').style.whiteSpace='nowrap'
  var p=document.createElement('p')
  p.id='testP'
  p.style.fontSize='25%' //fix for mozilla. multiply by 4 before using
  x-=step
  if (document.getElementById('tag').className) p.className=document.getElementById('tag').className
  p.appendChild(document.createTextNode(sText))
  document.body.appendChild(p)
  pw=p.offsetWidth
  document.body.removeChild(p)
  if (x<(pw*4)*-1){x=divW}
  document.getElementById('tag').style.left=x+'px'
  scroll=setTimeout('startScroller()',speed)
}

function initScroller(){
  if (document.getElementById && document.createElement && document.body.appendChild) {
    addControls();
    divW=document.getElementById('scroller').offsetWidth;
    x=divW;
    document.getElementById('tag').style.position='relative';
    document.getElementById('tag').style.left=divW+'px';
    var ss=document.getElementById('tag').childNodes;
    for (i=0;i<ss.length;i++) {sText+=ss[i].nodeValue+" "};
    scroll=setTimeout('startScroller()',speed);
  }
}


function addLoadEvent(func) {
  if (!document.getElementById | !document.getElementsByTagName) return
  var oldonload = window.onload
  if (typeof window.onload != 'function') {
    window.onload = func;
  } else {
    window.onload = function() {
      oldonload()
      func()
    }
  }
}

addLoadEvent(initScroller)

body {font:1em verdana,sans-serif; color:#000; margin:0}

/* position:relative and overflow:hidden are required */
#scroller { position:relative; overflow:hidden; width:30em; border:1px solid #008080; }

/* add formatting for the scrolling text */
#tag { margin:2px 0; }

/* #testP must also contain all text-sizing properties of #tag  */
#testP { visibility:hidden; position:absolute; white-space:nowrap; } 

/* used as a page top marker and to limit width */
#top { width:350px; margin:auto; }

<div id="scroller">
    <div id="tag">
        <img src="https://picsum.photos/1500/600/?image=1"/>
        <img src="https://picsum.photos/1500/600/?image=2"/>
    </div>
</div>

Answer

I got a bit lost in the given JS, and wonder if it is necessary for this relatively simple task.

Here is a method using HTML and CSS for the continuous scrolling and with JS just for the pause/play part.

Because you want continuous scrolling with no gap or jump when the sequence of images goes back to the beginning you need two copies. The tag element is scrolled to the left by half of its width which means that the set of images overwrite themselves so giving a smooth effect.

The JS for the button uses the running value and toggles that.

.playpause {
  top: 10%;
  left: 10%;
  position: absolute;
  z-index: 1;
}

#scroller {
  width: min(30em, 100vw);
  height: min(20em, 100vh);
  display: inline-block;
  white-space: nowrap;
  overflow: hidden;
  position: relative;
}

#scroller #tag {
  height: 100%;
  animation: scroll 10s linear infinite;
  animation-fill-mode: forwards;
  display: inline-block;
  font-size: 0;
}

@keyframes scroll {
  0% {
    transform: translateX(0);
  }
  100% {
    transform: translateX(-50%);
  }
}

img {
  height: 100%;
  width: auto;
  display: inline-block;
}

<div id="scroller">
  <button class="playpause" onclick="document.querySelector('#tag').style.animationPlayState = (document.querySelector('#tag').style.animationPlayState != 'paused') ? 'paused' : 'running';">PLAY/PAUSE</button>
  <div id="tag">
    <img src="https://picsum.photos/1500/600/?image=1" />
    <img src="https://picsum.photos/1500/600/?image=2" />
    <!-- second copy of all the imaages -->
    <img src="https://picsum.photos/1500/600/?image=1" />
    <img src="https://picsum.photos/1500/600/?image=2" />
  </div>
</div>

Observation: the site linked to in the question (gap) has a slightly unpleasant ‘jump’ half way through the images so I think they must be using a different method to achieve continuous scrolling.