Edit in JSFiddle

<ul class="flex-container flex-start">
  <li class="flex-item">1<br>2</li>
  <li class="flex-item">3</li>
  <li class="flex-item">4<br>5</li>
  <li class="flex-item">6</li>
</ul>

<ul class="flex-container flex-end">
  <li class="flex-item">1<br>2</li>
  <li class="flex-item">3</li>
  <li class="flex-item">4<br>5</li>
  <li class="flex-item">6</li>
</ul>

<ul class="flex-container center">
  <li class="flex-item">1<br>2</li>
  <li class="flex-item">3</li>
  <li class="flex-item">4<br>5</li>
  <li class="flex-item">6</li>
</ul>

<ul class="flex-container baseline">
  <li class="flex-item">1<br>2</li>
  <li class="flex-item">3</li>
  <li class="flex-item">4<br>5</li>
  <li class="flex-item">6</li>
</ul>

<ul class="flex-container stretch">
  <li class="flex-item">1<br>2</li>
  <li class="flex-item">3</li>
  <li class="flex-item">4<br>5</li>
  <li class="flex-item">6</li>
</ul>

.flex-container {
  padding: 0;
  margin: 0;
  list-style: none;
  -ms-box-orient: horizontal;
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -moz-flex;
  display: -webkit-flex;
  display: flex;
  float: left;
}

.flex-start {
  -webkit-align-items: flex-start;
  align-items: flex-start;
}

.flex-end {
  -webkit-align-items: flex-end;
  align-items: flex-end;
}

.flex-end li {
  background: gold;
}

.center {
  -webkit-align-items: center;
  align-items: center;
}

.center li {
  background: deepskyblue;
}

.baseline {
  -webkit-align-items: baseline;
  align-items: baseline;
}

.baseline li {
  background: lightgreen;
}

.stretch {
  -webkit-align-items: stretch;
  align-items: stretch;
}

.stretch li {
  background: hotpink;
}

.flex-item {
  background: tomato;
  padding: 5px;
  width: 50px;
  margin: 5px;
  line-height: 50px;
  color: white;
  font-weight: bold;
  font-size: 2em;
  text-align: center;
}