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The CSS border box is the rectangle bounded by the outer border edges in the CSS box model. It includes the content area, padding and borders, while excluding margins.^[ref]

For a simple unfragmented box, let the CSS content box be $120$ by $40$ CSS pixels, padding be $8$ pixels on every side, and borders be $2$ pixels on every side. The border dimensions areAdding six-pixel left and right margins would give a CSS margin box width of $152$, without changing the $140$-pixel CSS border box width.

The sizing edge matters when reversing this calculation. With box-sizing: border-box; width: 140px and the same padding and borders, the content width is $120$ pixels. With content-box sizing, a declaration width: 140px would instead produce a $160$-pixel CSS border box width, assuming no other constraint.

A CSS border box is a layout region, not a claim that every point inside it is opaque or that nothing paints outside it. Border style and color affect appearance, and CSS overflow concerns content extending beyond a box. Distinguishing geometry from painting avoids treating a screenshot's visible ink as a complete measurement of layout size.

The CSS box model describes the content, padding, border and margin regions used to lay out generated boxes. These define a CSS content box, CSS padding box, CSS border box and CSS margin box respectively. A box's declared width must be interpreted together with the applicable sizing rules.^[ref]

For a simple horizontal box with content width $w$, left/right padding $p_l,p_r$, border widths $b_l,b_r$ and margins $m_l,m_r$, the total margin-box width isWith $w=100$, padding ten on each side, borders two on each side and margins ten on each side, the total is 144 CSS pixels. Its border box is 124 pixels wide.

CSS box-sizing changes which area a width declaration sizes. With the same padding and borders but box-sizing: border-box; width: 100px, the border box is 100 pixels and the content width is 76 pixels in this example. Margins are still outside that declared border-box size.^[ref]

The example assumes ordinary nonnegative fixed horizontal values. Vertical margins can participate in CSS margin collapsing, and an inline element split across lines can generate several fragments. The box model supplies the regions; the formatting context determines how particular boxes are generated and positioned.

The CSS content box is the innermost box region bounded by the content edges, inside padding, border and margins. It is the area assigned for the box's content by the applicable layout rules.^[ref]

Suppose the content width is $120$ CSS pixels, with eight-pixel left/right padding and two-pixel left/right borders. Then the CSS padding box width is $136$ and the CSS border box width is $140$ pixels. Six-pixel margins on both sides increase the CSS margin box width to $152$ pixels. These are different measurements of the same simple box, not competing answers to one unspecified width question.

With box-sizing: content-box; width: 120px, the declared width requests that content dimension. With box-sizing: border-box; width: 120px and the same padding and borders, the requested border dimension instead leaves $120-16-4=100$ pixels for content, before any further constraints.

A content area is not a guarantee that all painted content fits inside it. A long unbroken item or a descendant can cause CSS overflow. Also, an ordinary inline element can fragment across CSS line boxes; its geometry should not be reduced to one fixed-width rectangle merely because a width declaration appears in its style. Read the content region together with CSS box-sizing, box kind and the layout context.

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]

The CSS margin box is the box bounded by the outer margin edges, outside the border, padding and content regions. It describes the margin contribution around a generated box. Margins are not part of the CSS border box sized by box-sizing: border-box.^[ref]

For a simple box with border-box width 100 CSS pixels and left/right margins of ten pixels each, the margin-box width is 120 pixels. A declaration width: 100px does not automatically include those margins, even when border-box sizing is selected.

The margin area is also not a painted extension of the element's background. Visible background and border geometry should therefore not be used to infer that all surrounding layout space belongs to the same painted area. Auto and negative margins have their specified layout effects as well, so a margin is not restricted to a fixed positive gap.

Vertical relationships require particular care. Two adjoining margins can participate in CSS margin collapsing, where the resulting separation is not simply their sum. For instance, eligible adjoining positive margins of 20 and 30 pixels collapse to 30 pixels under the applicable rules, rather than yielding 50.^[ref]

A margin-box calculation is consequently meaningful only together with the formatting context and the directions being considered. The uncomplicated horizontal sum illustrates box geometry; it is not a substitute for the separate vertical collapsing rules.

CSS margin collapsing combines eligible adjoining vertical margins into one collapsed margin. In the horizontal text layout described by CSS 2.2, this can occur between ordinary in-flow block boxes and in specified parent-child or empty-block situations. It does not mean that every pair of nearby margins is added or collapsed.^[ref]

For two eligible sibling blocks, a first block's bottom margin of $20$ CSS pixels and the next block's top margin of $30$ give a collapsed margin of $30$, not $50$. Positive adjoining margins contribute their largest value.

Negative values make the rule more interesting. For a collapsing group with a positive margin of $30$ and a negative margin of $-12$, the result is $30-12=18$ pixels. With only $-8$ and $-12$, it is $-12$, not $-20$. More generally, take the largest positive value, or zero if none, and subtract the largest absolute negative value, or zero if none.^[ref]

Eligibility comes before arithmetic. Horizontal margins do not collapse under these rules, and margins of a CSS inline-block box do not collapse with surrounding boxes or its in-flow children. A parent's top border or padding can prevent the top parent-child collapse that would otherwise occur. Establishing a separate CSS block formatting context also changes the relevant parent-child relationships.^[ref]

A useful debugging example is a parent's first child's top margin apparently moving the whole group rather than creating an internal gap. Inspect whether those margins are adjoining before changing unrelated dimensions. The CSS margin box describes a box's margin geometry, while CSS margin collapsing specifies how eligible margins interact during layout.