space

Space is the cheapest material and the one generated interfaces waste most. Uneven spacing is the first thing an experienced eye sees and the last thing a model fixes.

The scale

Everything sits on a 4px grid. The useful stops:

2 · 4 · 6 · 8 · 10 · 12 · 16 · 20 · 24 · 32 · 40 · 48 · 64 · 96

Choose a value from this list, never in between. If 14px "looks right", the neighbouring elements are wrong; fix them and 12 or 16 will look right.

Related things sit closer than unrelated things. A label to its input: 6. Two inputs in a form: 16. Two sections of a form: 32. The ratio between levels is what makes structure visible without lines.

Density

Decide it per surface, not per component:

Surface	Row height	Padding inside	Between groups
Dense list / table (tools, admin)	28–32px	8–10px horizontal	24px
Comfortable list (settings, feeds)	40–48px	12–16px	32px
Reading	—	0, the measure does the work	40–64px
Marketing	—	24–40px	96px+

A dense tool with 48px rows feels like a toy. A settings page with 28px rows feels like a spreadsheet. Match the density to how long someone looks at each item.

Widths

App frame: 1120–1280px max, centred, with a visible edge (border or background change) so the frame reads as an object on the page.
Reading measure: 55–70ch, set on the text block.
Sidebars: 240–280px. Narrower and labels truncate; wider and the content column suffers.
Forms: 400–520px. A full-width text input on a 1400px screen is a mistake.
Dialogs: 400px (confirm), 520px (form), 720px (content). Never full-width on desktop.

Gap, not margin

Space between siblings is the parent's job. Use gap in flex and grid layouts; children carry no outer margin. It removes the "last child has extra margin" class and the "first child is too high" bug, and it means a component can be dropped anywhere without bringing its spacing with it.

Margins are for one thing: separating a block from what follows in flowing text (margin-bottom on paragraphs in an article).

Alignment

Text aligns to text: the left edge of a title, its description and the rows beneath share one x. Icons sit outside that line, not inside it.
Icons and text on a row align on the icon's optical centre, which is usually 1px above the geometric one. Nudge with translateY(-0.5px) or margin-top: -1px, not with padding on the text.
Numbers right-align in columns; text left-aligns; never centre a column of anything you compare.
Buttons in a row share a baseline and a height. A primary and a secondary button never differ in height.
Optical alignment beats geometric: a play icon looks centred when it is slightly right of centre; a rounded pill's text looks centred when it is slightly left.

Rhythm

Vertical spacing between sections increases with the level of the break:

Rows inside a section: 0 (rows are the spacing) or 2–4px
Section header to its rows: 8–12px
Between sections: 24–32px
Page header to first section: 28–40px

Consistent rhythm is what makes a long page feel calm. Vary content, not spacing.

Responsive

Fluid first, breakpoints second:

Let the content wrap and the grid collapse (repeat(auto-fill, minmax(280px, 1fr))) before adding any media query.
Two or three breakpoints at most, tied to what breaks: sidebar collapses (~1024px), columns stack (~640px). Not "sm/md/lg/xl/2xl" because the framework has them.
Use container queries for components that live in different-width slots. A card should not know the viewport.
Side gutters: 16px on phones, 20–28px on desktop. Never 0.
Nothing scrolls horizontally except things meant to (a carousel, a code block with overflow-x: auto).

Test at 375px, 768px, 1024px and 1440px. Then resize slowly between them and watch for jumps.

Sticky and fixed

A sticky header is 44–52px, translucent (background: color-mix(...) at 80% + backdrop-filter: blur), with a bottom border. Content beneath uses scroll-margin-top matching the height so anchors land below it.
Sticky sidebars are position: sticky; top: 0; height: 100dvh with their own scroll region for overflow, using overscroll-behavior: contain.
Fixed bottom bars on phones sit above the home indicator: padding-bottom: env(safe-area-inset-bottom).
Nothing is fixed on top of reading content on a phone except a bar under 56px.

Scroll containers

Inner scroll regions are explicit (min-height: 0 on the flex child, overflow-y: auto), have a visible edge, and get overscroll-behavior: contain.
Only one axis scrolls at a time in any region.
A scroll region that is not obviously scrollable needs a cue: a fade at the bottom edge, or content visibly cut at the boundary.

Empty space as a tool

A page with one form and a lot of space around it reads as focused. The same form crammed against a sidebar and a header reads as cluttered. Before adding a card border to group things, try more space around the group and less inside it.

When an area feels empty and wrong, it is usually the width, not the space: the content is narrower than the region it sits in. Constrain the region, do not fill it.

Grids for cards

grid-template-columns: repeat(auto-fill, minmax(280px, 1fr)) with gap: 8–16px. Cards share one height per row (align-items: stretch) and the content inside aligns top, with actions pushed to the bottom by margin-top: auto. Never masonry for anything with actions.

Marks of unconsidered space

Sign	Fix
Values like 13px, 18px, 22px, 30px	Snap to the scale
Same 16px gap between everything	Hierarchy: 8 inside, 16 between, 32 between groups
Margins on components	`gap` on the parent
Card border to group three things	Space around them
Full-width inputs on desktop	400–520px
Five breakpoints	Two, where it actually breaks
Icon vertically centred by padding	Align on optical centre, nudge 1px
Content jumping at a breakpoint	Fluid grid, then fewer breakpoints

From the catalogue

Ahmad Shadeed — deep CSS layout write-ups with interactive figures.
Every Layout — the layout primitives, composed.
Utopia — fluid space and type scales.
Refactoring UI — the spacing-hierarchy argument in pictures.
Refero, Mobbin — see how shipped apps set density.
More at stealth.pm/learn.