What\'s the difference between \'align-items\' and \'align-content\' in CSS Grid Layout?

Understanding the Distinction between "align-items" and "align-content" in Grid Layout

The Grid Layout module introduces two sets of properties, "justify/align-items" and "justify/align-content," which play distinct roles in aligning grid items and the grid itself within a grid container.

Grid Terminology Clarification

• Grid Container: The parent container that defines the overall grid space.
• Grid: A structured grid of lines that divides the grid container into grid areas.
• Grid Items: Boxes that contain in-flow content within the grid areas.

Align-items vs. Align-content

The "align-items" and "align-content" properties, as their names suggest, align grid items and the grid, respectively. Specifically:

• justify-content and align-content align the grid tracks within the content box of the grid container.
• justify-self and justify-items align grid items in the inline dimension (horizontal by default).
• align-self and align-items align grid items in the block dimension (vertical by default).

Addressing the Author's Claim

The author's claim that the "-content" properties exist because "sometimes the total size of your grid might be less than the size of its grid container" highlights the following:

• When the grid is smaller than the grid container, there is free space available.
• The "justify-content" and "align-content" properties can utilize this space to align the grid centrally or otherwise within the container.
• In contrast, if the grid size equals the container size, there is no free space, and these alignment properties have no effect.

Illustrations for Clarity

[Image of illustration from W3C showing a grid smaller than its container, with "justify-content" and "align-content" aligning the grid within the container.]

Excerpt from the Specification

For clarity, relevant excerpts from the CSS Grid Layout specification are provided:

• "Grid items can be aligned in the inline dimension by using the justify-self property on the grid item or justify-items property on the grid container."
• "Grid items can also be aligned in the block dimension by using the align-self property on the grid item or align-items property on the grid container."
• "If the grid’s outer edges do not correspond to the grid container’s content edges, the grid tracks are aligned within the content box according to the justify-content and align-content properties on the grid container."

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