Je réserve

HTML

<div id="contenu">
<form>
<fieldset>
<button type="button" id="moins" onclick="doLess();">
<svg viewbox="0 0 28 28" stroke-width="2">
<circle cx="14" cy="14" r="13"/>
<line x1="10" y1="14" x2="18" y2="14"/>
</svg>
</button>
<div id="num">
<div id="innum">
<span>1</span>
</div>
</div>
<button type="button" id="plus" onclick="doMore();">
<svg viewbox="0 0 28 28" stroke-width="2">
<circle cx="14" cy="14" r="13"/>
<line x1="9" y1="14" x2="19" y2="14"/>
<line x1="14" y1="9" x2="14" y2="19"/>
</svg>
</button>
<button type="button" id="fois" onclick="doReserve();">
<svg viewbox="0 0 28 28" stroke-width="2">
<circle cx="14" cy="14" r="13"/>
<line x1="9" y1="9" x2="19" y2="19"/>
<line x1="9" y1="19" x2="19" y2="9"/>
</svg>
</button>
</fieldset>
<fieldset>
<input type="hidden" name="num" value="1">
<input type="submit" value="Je réserve">
</fieldset>
</form>
</div>

CSS

body {
  margin: 0;
  padding: 0;
}

#contenu {
  width: 16rem;
  height: 4rem;
  margin: 0 auto;
  background: #eea595;
  box-shadow: 0 0.5rem 0.25rem #eaeaea;
}

form {
  font-family: arial, sans-serif;
  position: absolute;
  left: calc(50% - 8rem);
  top: 2.5rem;
  width: 16rem;
  height: 5rem;
  margin: 0;
  padding: 0;
  overflow: hidden;
}

fieldset {
  border: 0;
  margin: 0;
  padding: 0;
  position: absolute;
  display: block;
  left: 0;
  right: 0;
  height: 5rem;
  transition: top 0.5s ease;
}

fieldset:first-of-type {
  top: 0;
}

fieldset:last-of-type {
  top: -5rem;
}

[type="submit"],
button {
  display: block;
  box-sizing: content-box;
  font-size: 1rem;
  font-family: arial, sans-serif;
  position: absolute;
  font-weight: bold;
  text-align: center;
  padding: 0;
  margin: 0;
}

[type="submit"]:focus,
button:focus {
  outline: 0;
}

[type="submit"]:hover,
button:hover {
  cursor: pointer;
}

[id="moins"],
[id="plus"] {
  background: transparent;
  border: 0;
  width: 1.75rem;
  height: 1.75rem;
  top: 0.5875rem;
}

[id="moins"] {
  left: 20%;
}

[id="plus"] {
  right: 20%;
}

[id="fois"] {
  background: transparent;
  border: 0;
  height: 1.25rem;
  width: 1.25rem;
  top: 3.75rem;
  left: calc(50% - 0.625rem);
}

svg {
  width: 100%;
  height: 100%;
}

[id="moins"] svg,
[id="plus"] svg {
  fill: #fff;
  stroke: #fe9566;
}

[id="fois"] svg {
  fill: #fe9566;
  stroke: #fff;
}

[type="submit"] {
  height: 3rem;
  line-height: 3rem;
  text-transform: uppercase;
  width: calc(100% - 2rem);
  margin: 0;
  top: 0;
  left: 1rem;
  border: 0.125rem solid #fe9566;
  border-radius: 1.5rem;
  color: #fe9566;
  background: #fff;
  box-shadow: 0 0.25rem 0.125rem #e0e0e0;
}

#num {
  position: absolute;
  top: 0;
  left: calc(50% - 1.5rem);
  width: 3rem;
  height: 3rem;
  line-height: 3rem;
  border-radius: 1.5rem;
  background: #fe9566;
  color: #fff;
  border: 0;
  box-shadow: 0 0.25rem 0.125rem #e0e0e0;
  overflow: hidden;
}

#innum {
  position:...

JavaScript

var num = 1;

function doReserve() {
  document.querySelector('fieldset:first-of-type').style.top = "5rem";
  document.querySelector('fieldset:last-of-type').style.top = "0";
}

function doLess() {
  num--;
  if (num < 1) num = 1;
  document.querySelector('[id="innum"]').style.top = (-(num - 1) * 3) + "rem";
  document.querySelector('[name="num"]').value = num + "";
}

function doMore() {
  num++;
  if (!document.querySelector('[id="innum"] span:nth-of-type(' + num + ')')) {
    let e;
    e = document.createElement("span");
    e.innerHTML = num + "";
    document.querySelector('[id="innum"]').appendChild(e);
  }
  document.querySelector('[id="innum"]').style.top = (-(num - 1) * 3) + "rem";
  document.querySelector('[name="num"]').value = num + "";
}