What’s the difference between flex-start and baseline?

See the following two images. The code is identical for both, except for the align-items rule.

align-items: flex-start

align-items: baseline

When using align-items or align-self, the flex-start value will align flex items at the starting edge of the cross-axis of the flex container.

The baseline value will align flex items along their content’s baseline.

baseline

The line upon which most letters “sit” and below which descenders extend.

^{Source: Wikipedia.org}

In many cases, when the font size is the same among items (like in the question), or the content is otherwise the same, then flex-start and baseline will be indistinguishable.

But if content size varies among flex items, then baseline can make a noticeable difference.

In terms of where baseline alignment occurs, that is determined by the tallest item in the row.

From the spec:

8.3. Cross-axis Alignment: the align-items and align-self
properties

baseline

The flex item participates in baseline alignment: all participating flex items on the line are aligned such that their baselines align, and the item with the largest distance between its baseline and its cross-start margin edge is placed flush against the cross-start edge of the line.

.flex-container {
  color: white;
  display: flex;
  height: 300px;
  background-color: yellow;
  justify-content: space-between;
  align-items: baseline;
}
.flex-item {
  background-color: green;
  width: 110px;
  min-height: 100px;
  margin: 10px;
  box-sizing: border-box;
  padding: 5px;
  text-align: center;
}
.item1 {  font-size: 2em;  }
.item2 {  font-size: 7em;  }
.item3 {  font-size: .5em; }
.item4 {  font-size: 3em;  }
.item5 {  font-size: 10em; }

/*
.item1 {  align-self: flex-start; }
.item2 {  align-self: flex-end; }
.item3 {  align-self: center; }
.item4 {  align-self: baseline; }
.item5 {  align-self: stretch; }
*/

#dashed-line {
  border: 1px dashed red;
  position: absolute;
  width: 100%;
  top: 170px;
}

<div class="flex-container">
  <div class="flex-item item1">A</div>
  <div class="flex-item item2">B</div>
  <div class="flex-item item3">C</div>
  <div class="flex-item item4">D</div>
  <div class="flex-item item5">E</div>
</div>

<div id="dashed-line"></div>

jsFiddle version