Flexbox `align-items`: `flex-start` vs. `baseline` – When Should I Use Which?

Understanding the Distinction Between flex-start and baseline

When leveraging flex -align properties, the differentiation between flex-start and baseline may not be immediately apparent. This is because, in many situations involving consistent font sizes or equivalent content, both values appear to yield the same alignment. However, there are crucial distinctions that become evident when dealing with varying content sizes.

flex-start Alignment

The flex-start value aligns flex items at the starting edge of the cross-axis within the flex container. This alignment typically aligns the left edges of flex items in horizontal layouts and the top edges in vertical layouts.

baseline Alignment

In contrast, the baseline value aligns flex items according to their content's baseline. The baseline refers to the line where most letters and characters rest, with descenders extending below it. Flex items are positioned such that their baselines align, with the item featuring the largest distance between its baseline and the cross-start margin edge being placed flush against the cross-start edge.

Differences in Application

The choice between flex-start and baseline alignment depends on the intended effect. Flex-start is suitable when aligning elements with consistent sizes. On the other hand, baseline alignment is particularly useful when handling elements with varying font sizes, as it ensures that the baseline is the same across all items.

Example Demonstration

Consider the following code snippet:

.flex-container {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  height: 300px;
  background-color: yellow;
}
.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: 0.5em;
}
.item4 {
  font-size: 3em;
}
.item5 {
  font-size: 10em;
}

With baseline alignment applied, the flex items align along their respective baselines. Notice how the alignment is influenced by the tallest item (item 5), as per the specification. A dashed line represents the baseline across the flex items.

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