Use when laying out a section that is two-dimensional (rows AND columns matter at the same time), when nested grid items need to align to lines defined on an outer grid, when card titles or meta rows in a card list refuse to line up across columns, when a responsive grid must auto-fit or auto-fill an unknown number of items, when negative margins are being considered to "pull" a child into a parent track, or when a layout needs named lines or named areas that survive refactors. Prevents the classic "nested grid with hand-tuned margins" anti-pattern, the `auto-fill` versus `auto-fit` confusion that collapses or preserves empty tracks unpredictably, declaring subgrid on both axes when implicit rows are still needed, assuming masonry layout is Baseline (it is not), and using the legacy `grid-gap` property name instead of `gap`. Covers `display: grid` and `display: inline-grid`, `grid-template-columns` and `grid-template-rows`, `grid-template-areas` with named areas and dot null cells, named lines with `[name]` b
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Use when laying out a section that is two-dimensional (rows AND columns matter at the same time), when nested grid items need to align to lines defined on an outer grid, when card titles or meta rows in a card list refuse to line up across columns, when a responsive grid must auto-fit or auto-fill an unknown number of items, when negative margins are being considered to "pull" a child into a parent track, or when a layout needs named lines or named areas that survive refactors. Prevents the classic "nested grid with hand-tuned margins" anti-pattern, the `auto-fill` versus `auto-fit` confusion that collapses or preserves empty tracks unpredictably, declaring subgrid on both axes when implicit rows are still needed, assuming masonry layout is Baseline (it is not), and using the legacy `grid-gap` property name instead of `gap`. Covers `display: grid` and `display: inline-grid`, `grid-template-columns` and `grid-template-rows`, `grid-template-areas` with named areas and dot null cells, named lines with `[name]` brackets, `repeat()` with `auto-fill` and `auto-fit`, `minmax()`, `fit-content()`, the `fr` unit, `gap` and `row-gap` and `column-gap`, implicit tracks with `grid-auto-rows` and `grid-auto-columns`, `grid-auto-flow` with `row` and `column` and `dense`, item placement with `grid-column` and `grid-row` and `grid-area`, alignment with `justify-items` and `align-items` and `place-items` and `justify-self` and `align-self`, and `subgrid` on either or both axes (Baseline Widely since September 2023) with named-line passthrough, gap inheritance, and the no-implicit-tracks hard limitation. Keywords: display grid, inline-grid, grid-template-columns, grid-template-rows, grid-template-areas, subgrid, named lines, named-line passthrough, repeat, minmax, fit-content, auto-fill, auto-fit, fr unit, gap, row-gap, column-gap, grid-auto-rows, grid-auto-columns, grid-auto-flow, dense, place-items, justify-self, align-self, grid-area, card layout aligned baselines, layout breaks at sub-component, items misaligned, gap not working, grid tracks do not align across rows, subgrid not working, implicit rows in subgrid not generated, nested items not aligned to outer grid, how to align nested grid items, what is subgrid, when to use grid vs flexbox, how do I make a card layout with aligned baselines, why is grid-gap not working, when to use auto-fill vs auto-fit, how do named lines work.
license
MIT
compatibility
Designed for Claude Code. Requires Frontend Design evergreen-2026.
metadata
{"author":"OpenAEC-Foundation","version":"1.0"}
Frontend Syntax CSS Grid + Subgrid
This skill defines the CSS Grid layout surface and the rule for picking Grid versus Flexbox, and for picking subgrid versus a nested grid. CSS Grid is Baseline Widely Available since October 2017; subgrid is Baseline Widely Available since September 2023. The skill builds on [[frontend-core-architecture]] (style layer) and [[frontend-core-web-standards-baseline]] (Baseline gating).
Key rules verified against MDN: Subgrid (verified 2026-05-19):
subgrid is a value for grid-template-columns and/or grid-template-rows. It can be applied to one axis or both.
The subgrid spans EXACTLY the number of tracks its grid area covers on the parent.
Line numbering RESTARTS inside the subgrid (line 1 of subgrid is the parent line where the subgrid begins).
Parent named lines pass through the subgrid automatically and can be used to position children inside it.
The subgrid MAY add extra names after the subgrid keyword: grid-template-columns: subgrid [sub-a] [sub-b].
Gap (gap, row-gap, column-gap) is INHERITED from the parent on the subgridded axis and MAY be overridden on the subgrid.
HARD LIMIT: a subgridded axis CANNOT generate implicit tracks. If both axes are subgrid and there are more items than cells, excess items overflow into the last track. To allow implicit rows, declare subgrid on the column axis ONLY and use grid-auto-rows for rows.
Decision Trees
Decision: Grid or Flexbox?
What does the layout need to express?
Two-dimensional. Rows AND columns must align at the same time (gallery,
dashboard, app shell, card list where titles must align across columns).
-> CSS Grid on the outer container.
One-dimensional row OR column with intrinsic content sizing
(toolbar, breadcrumb trail, tag list, single-axis form row).
-> Flexbox on the container.
Both, nested. Outer layout is two-dimensional, but each cell is a
one-dimensional cluster (card with horizontal action row at the bottom).
-> Grid on the outer container. Flexbox inside the cell.
Rule of thumb: if the screenshot would show a clear column rhythm AND a clear row rhythm at the same time, the answer is Grid.
Decision: subgrid or nested grid?
Does the inner element need to align to the OUTER grid's lines?
No. The inner element has its own internal layout independent of the
outer track sizes.
-> Nested independent grid. Use `display: grid` with its own
grid-template-columns / grid-template-rows.
Yes, in BOTH axes, and the number of inner items is fixed and equal to
outer cells (e.g. a known 3x2 mini-grid).
-> `grid-template-columns: subgrid; grid-template-rows: subgrid;`.
Yes, in BOTH axes, but inner content has a variable number of rows
(cards with optional meta rows).
-> `grid-template-columns: subgrid;` ONLY. Use `grid-auto-rows` for
inner rows. Subgridding both axes here is the implicit-track trap.
Yes, in ONE axis only (card titles aligned across columns, free row
count per card).
-> Subgrid on the relevant axis only.
Decision: auto-fill or auto-fit?
What should happen when the container is wider than (item count x min track)?
Reserve space for ghost tracks; existing items DO NOT stretch beyond their
declared max. Useful when the grid is partially filled and you want a
predictable column rhythm regardless of item count.
-> repeat(auto-fill, minmax(<min>, 1fr))
Collapse empty tracks; existing items stretch to fill the row width.
Useful when you want the visible row to look "complete" with any item count.
-> repeat(auto-fit, minmax(<min>, 1fr))
If the column count is fixed and known, use repeat(<N>, 1fr) and skip the auto-* keyword entirely.
Decision: named lines or grid-template-areas?
Is the layout LITERATE in column count and shape?
The layout is a recognisable named shape (header / sidebar / main / footer,
hero / aside / content). Authors will edit the layout by adding or moving
named areas.
-> grid-template-areas. Areas form rectangles; place items with
grid-area: <name>.
The layout is a column rhythm with content that flows by position
(e.g. 12-column page grid, prose narrow column, figures full-bleed).
-> Named lines with [name] brackets. Place items with grid-column.
Both. Areas for the page shell, named lines inside each area.
-> Combine. Areas auto-generate <name>-start / <name>-end lines that
inner items can target.
Patterns
Pattern 1: card list with aligned baselines (subgrid)
Outer is a 3-column track grid; each card is a 3-row subgrid (title, body, meta), so titles across cards align, bodies align, meta rows align.
Masonry is still a draft proposal in the CSS Grid Layout specification and is NOT Baseline as of 2026. Verified against W3C: CSS Grid Layout Module Level 2 (verified 2026-05-19): masonry is not part of the published Module Level 2 surface. ALWAYS gate masonry behind @supports (grid-template-rows: masonry) and ship a non-masonry fallback as the default rule. NEVER assume masonry is usable in evergreen-2026 production.
Anti-Patterns Index
See anti-patterns.md for the full list. Minimum five patterns covered: subgrid on both axes with implicit-row needs, auto-fit versus auto-fill confusion, nested grid without subgrid expecting alignment, negative margin to align child to parent track, masonry assumed Baseline, grid-gap legacy name, grid-auto-flow: dense reordering reading order (accessibility issue), display: grid on inline elements without display: inline-grid.
Reference Links
Methods and signatures : complete property and function table for grid containers, items, and subgrid.
Examples : working HTML/CSS snippets for the five patterns above plus subgrid edge cases.
Anti-patterns : eight cataloged anti-patterns with symptom, root cause, and fix.
Cross-references
[[frontend-syntax-css-container-queries]] : when "the same component must reflow in different parent widths", combine subgrid with container queries.
[[frontend-impl-responsive-layout-fluid]] : fluid grids using clamp() and minmax().
[[frontend-core-architecture]] : where Grid sits in the style layer (W3C CSSWG).
[[frontend-core-web-standards-baseline]] : Baseline gating for subgrid and masonry.
