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A CSS inline-block box participates as one unit in its parent's CSS inline formatting context while providing a CSS block container for its own contents. It can therefore sit beside text or other inline-level boxes while containing independently laid-out text and blocks. The outer and inner layout roles are separate.^[ref]

For example, two cards each have a CSS border box width of $80$ CSS pixels, zero margins and an intervening space that occupies $10$ pixels. Their combined width is $80+10+80=170$ pixels. They can share an available line width of $180$ pixels, but not $160$ pixels under these assumptions. Text inside either card can wrap within that card; the surrounding line treats the card as one unit instead of dividing it into several fragments.

Declaring display: inline-block; width: 80px also makes width applicable to an ordinary non-replaced element that would otherwise have generated an inline box. Check CSS box-sizing before deciding whether the eighty pixels describe its content or border dimensions.

CSS vertical alignment determines placement within the surrounding line. A gap beneath an inline-block can consequently concern baseline alignment rather than a mysterious extra margin. Source whitespace between cards can also occupy inline space. Diagnose these effects using the outer line layout and the card's internal CSS block formatting context separately.

A CSS inline formatting context lays out inline-level content along lines. The generated CSS inline boxes and other inline-level items are arranged into CSS line boxes, whose widths depend on the available space and whose heights account for text metrics and vertical alignment.^[ref]

For a synthetic wrapping example, suppose three words each occupy 60 pixels and the spaces between them occupy ten. Two words need 130 pixels; all three need 200. With 140 pixels available and ordinary breaking opportunities at the spaces, two fit on the first line and the third moves to another. With 220 pixels available, all three can fit together.

One HTML element can then generate fragments on several lines. A styled span containing a long phrase need not correspond to one unbroken rectangular painted box. Conversely, an inline replaced image or a CSS inline-block box can participate as a single inline-level item whose own dimensions affect the line.

Vertical placement is handled within that context. CSS vertical alignment and text metrics can change a line's required extent even when its words and available horizontal width remain unchanged. The inline formatting context therefore combines line breaking with placement; knowing only the element tree or only the font size does not fully specify the result.

A CSS line box is the rectangular region containing the inline content of one line in a CSS inline formatting context. The browser fits successive inline content into lines, using line breaking when necessary, then uses CSS vertical alignment to determine each line's extent. A CSS inline box supplies one contribution to this enclosing region.^[ref]

For ordinary baseline-aligned text, the CSS leading calculation supplies each contribution's height $a_i$ above and depth $d_i$ below the text baseline. Include the CSS strut among those contributions. Enclosing them requiresFor example, extents $(a_1,d_1)=(10,4)$ and $(a_2,d_2)=(17,3)$ CSS pixels produce a line of $17+4=21$ CSS pixels, although the individual extents sum to 14 and 20. The largest upper extent and largest lower extent can come from different contributions. The CSS leading diagram illustrates the same geometry with another pair of hypothetical metrics.

A CSS replaced element, such as an inline image, contributes according to its CSS margin box; it need not follow the text's line height. Other settings of CSS vertical alignment require additional positioning rules. These effects explain why a line's final height can exceed the line-height declared on its surrounding CSS block container.^[ref]

A CSS replaced element represents content whose internal rendering lies outside the ordinary CSS formatting of the element's descendants. An image element displaying its image is a standard example. CSS can size and position the element's surrounding box, while the image's own dimensions or aspect ratio can help determine that size.^[ref]

Suppose an image has intrinsic dimensions $800\times 400$ and no extra padding or borders. With width: 200px; height: auto, preserving its intrinsic ratio gives a content height of 100 CSS pixels. The source image's pixel counts and the rendered CSS dimensions therefore have distinct roles. Choosing both dimensions explicitly can instead impose a different box ratio.

Replaced content is not synonymous with block layout. The same image can participate as an inline-level box in a line or be given a block-level display role. The distinction also matters to CSS vertical alignment, because an inline replaced box is aligned as a unit rather than as a run of ordinary text.

A broken or unavailable image can have specified fallback behavior, so an element's tag name alone is not a complete description of every rendering state. For a concrete sizing calculation, state what content is being replaced, which intrinsic information is available and which CSS dimensions constrain the resulting box.

CSS vertical alignment concerns the vertical-align property in its applicable formatting contexts. For inline-level content, it aligns boxes relative to the line and neighboring text metrics; on table cells, it controls alignment of cell content under the table rules. The relevant reference is therefore determined by context.^[ref]

For an inline image with zero margins, vertical-align: middle aligns the image's vertical midpoint with the parent's text baseline plus half the parent's x-height. It does not generally place the midpoint on the baseline itself.

A numerical example makes that offset visible. Measure height upward from a parent baseline at zero, and let the parent's x-height be ten pixels. A 20-pixel-high image aligned middle has its centre at $5$, its top at $15$ and its bottom at $-5$ CSS pixels. Aligning its centre at zero would produce a different placement.

A positive length such as vertical-align: 4px raises the applicable inline box from its baseline-aligned position; keywords such as top and bottom use specified line-box relationships instead. These placements contribute to the extent required by a CSS line box, so changing alignment can affect the line as well as one item.

To interpret a result, identify the element's generated box, its margins and the surrounding text metrics. CSS vertical alignment is not a general instruction that centers any arbitrary child inside any arbitrary block container.