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CSS scroll margin supplies outsets around an element's transformed border bounds to define its CSS scroll snap area. The scroll-margin shorthand assigns top, right, bottom and left values using the usual one-to-four-value ordering. In the pinned specification these values are lengths, rather than percentages.^[ref]

For an own example, let an untransformed CSS border box occupy $[200,300]\times [100,150]$ in content coordinates measured in CSS pixels. Set scroll-margin: 20px 10px;. The first value applies at top and bottom, and the second at left and right, giving the snap areaThe box itself is still $100$ by $50$ pixels; the alignment region is $120$ by $90$.

Suppose a reachable horizontal start alignment should place this area's left edge at coordinate $30$ within the CSS snapport. With a left-to-right horizontal convention, the candidate offset satisfies $190-s_x=30$, giving $s_x=160$. Without the left outset, it would instead be $200-30=170$. The extra target-side room changes the alignment computation without changing where the border is laid out in the content.

The construction uses the axis-aligned bounding box of the transformed border box before adding the outsets. A rotated CSS transform can therefore affect the base rectangle, even though the resulting snap area remains rectangular in the scroll container's coordinates.^[ref]

CSS scroll margin and ordinary layout margins serve different purposes. The former does not assign this extra space between neighboring elements in normal layout. It is also distinct from CSS scroll padding, which adjusts the viewing/alignment region on the container side.

The specification recommends using the snap area for fragment targets and scrollIntoView() targeting even when snapping is disabled or does not apply to the element. The property can consequently express useful target-side breathing room beyond a mandatory snapping interaction.^[ref]

CSS scroll padding defines inward offsets from a CSS scrollport to establish an optimal viewing region. In a snap container that region is the CSS snapport, used to align CSS scroll snap areas. The scroll-padding shorthand sets its sides, while individual longhands can control one side.^[ref]

For an own example, take a $400$-by-$200$ CSS pixel scrollport and set scroll-padding: 10%;. Percentages resolve against the corresponding scrollport dimension: left and right are $40$ pixels each, top and bottom $20$ each. In physical coordinates the optimal region iswith dimensions $320$ by $160$. The declaration gives the same percentage on each side, not the same absolute number of pixels.^[ref]

An explicit scroll-padding-top: 50px can reserve room in the preferred viewing region when an overlay obscures the top of a scrolling panel. This does not itself create the overlay or change the content's normal layout. It tells scrolling operations where a useful visible alignment should lie.

The initial value auto leaves the offset to the user agent. The pinned specification says it should generally resolve to zero, while allowing heuristics that choose a nonzero value in appropriate cases. Code that needs the example's explicit geometry should therefore state its offsets instead of silently treating every automatic value as zero.^[ref]

These offsets do not clip the excluded strip, resize the scrollport, change the CSS scroll origin or redefine the CSS initial scroll position. They affect the optimal region for operations such as targeting, focusing and paging; the property applies to scroll containers even when they are not snap containers.^[ref]

CSS scroll margin describes outsets on the target side, whereas CSS scroll padding describes insets on the container side. Using both requires aligning the resulting two regions rather than adding ordinary box-model padding to the element and assuming it has the same effect.

The CSS snapport is the alignment-container region used when CSS scroll snapping aligns a target's CSS scroll snap area. It is derived from the CSS scrollport by the inward offsets specified through CSS scroll padding. This lets snapping use a preferred viewing region rather than always aligning to the scrollport's outer edges.^[ref]

For an own two-dimensional example, take a $300$-by-$200$ CSS pixel scrollport, with physical coordinates increasing rightward and downward. Declare scroll-padding: 10px 20px 30px 40px;, in top/right/bottom/left order. The snapport iswith width $300-40-20=240$ and height $200-10-30=160$. The scrollport itself remains $300$ by $200$.

Suppose a target's snap area begins at content coordinate $x=500$ and is $100$ pixels wide, with no scroll margin. Assume horizontal left-to-right layout, no transforms and an adequate reachable scrolling range. For start alignment, its displayed left edge must equal the snapport's left edge:With zero left scroll padding, the same candidate would instead have $s_x=500$. The inset changes the alignment target without changing the element's position in its content layout.

An inset can accommodate a toolbar obscuring part of the viewing region, or provide space around a focused target. It does not itself clip the excluded region, change the CSS scroll origin or resize the content. Ordinary box padding and CSS scroll padding consequently have different effects.^[ref]

The alignment must also be interpreted with the appropriate CSS writing mode and reachable offsets. Start, end and center are alignments between the CSS scroll snap area and the CSS snapport, while selection among candidates belongs to the snapping rules. The numerical example solves one candidate; it does not predict which gesture will select it or prescribe an animation.^[ref]