Writing a block

A block is @@( … ), and what goes inside it is CSS.

A block is a value

So it goes wherever a value goes — in the attribute, or in a binding you name and use later:

const card = <div css={@@( display: flex; )}>inline</div>;

const panel = @@( display: flex; );
const named = <div css={panel}>named, and reusable</div>;

Name it when it is long, when two elements share it, or when you want to compose it into another.

Nothing about a block requires JSX. const panel = @@( … ) is a value like any other, and a block in a .ts file with no markup in it works the same way — this extends TypeScript, not JSX.

A block at module scope reads its holes ONCE

A block is a value, and a value at module scope is built when the module loads. So a hole in one is read at import time and then frozen:

let theme = "#10b981";

// Every element using this carries #10b981 for the life of the page, even
// after `theme` changes. The hole was read when this module loaded.
export const panel = @@( border-left: 4px solid {theme}; );

What the block compiles to is _merge({"border-left": ["r-…", theme]}) — an ordinary expression in an ordinary initialiser, so it is evaluated once, when the module loads. Nothing reports this.

It bites hardest where a shared block is most tempting: a default somebody imports everywhere. That is the one place a stale value spreads across the whole app rather than one component.

Three shapes that do not have the problem:

// 1. No hole — nothing to freeze, and the class is shared by every element that names it.
export const panel = @@( display: flex; gap: 8px; );

// 2. A function, so the hole is read per call.
export const panelFor = (theme: string) => @@( border-left: 4px solid {theme}; );

// 3. A custom property, which is what a value the whole app shares should be anyway.
export const panel2 = @@( border-left: 4px solid var(--theme); );

The third is the real answer for a theme, and it is cheaper as well as correct — see a hole is not a theme, where the same choice is measured at 41 bytes on every element.

Each declaration becomes one class

const row = <div css={@@( display: flex; gap: 8px; )}>x</div>;

ships as

<div class="r-disp-flex r-gap-8px">x</div>

Two files that write display: flex get the same class, without knowing about each other, because the name is derived from the declaration and from nothing else. That is also what makes composing possible: merging two blocks keeps, per thing set, the one written later — and it can only do that if each thing set has a class of its own to keep or drop.

Each file carries the rules it names in its own stylesheet, so a code-split route stands on its own. Where two files produce identical stylesheets the bundler dedupes them by content, and it costs nothing.

& is CSS, not ours

& means the thing this rule is nested in. It is CSS Nesting, standardised in 2023 and in every current browser, and the browser resolves it — nothing here rewrites it.

const card = @@(
  padding: 12px;
  &:hover { background: #f8fafc; }
  &.active { border-color: #10b981; }
  & .title { font-weight: 600; }
);

Two of CSS's own rules surprise people, so they are worth saying out loud:

• The space matters. & .title is a descendant; &.title is the same element carrying both classes. One character, two different rules.
• A prelude that names no parent gets one. div { … } inside a block means & div, which is what CSS Nesting says a bare selector means. The two compile to the same class.

Holes, for the values that change

A { … } hole carries a TypeScript expression into the CSS. Each one becomes one CSS custom property on the element, so a value that differs per instance costs a property rather than a rule:

class Row extends Component {
  @state weight = 4;

  render() {
    return (
      <div css={@@(
        border-left: {`${this.weight}px`} solid #ff0055;
        &:hover { border-left-color: #00b37e; }
      )}>
        a row
      </div>
    );
  }
}

The expression stays where you wrote it, so this.weight is the field beside it and is type-checked in that scope.

A hole holds a value, and only a value

border-left: {width};             ✓  a value
{name}: 24px;                     ✗  a property name
&:{state} { … }                   ✗  a selector
{on ? "display:flex" : ""}        ✗  a whole declaration

The last is refused rather than mangled. A value carrying a ; is refused outright — on the server too, where it would otherwise become real declarations in the markup.

There is one exception to the second line: a name that came from @@property( … ) may stand where a property name goes, because the compiler generated that name and nothing else can write it. See names the whole stylesheet sees.

The unit goes inside the hole

padding-left: {`${n}px`};          ✓  the hole carries its own unit
padding-left: calc({n} * 1px);     ✓  the arithmetic is CSS's
padding-left: {n}px;               ✗  reported

A var() is substituted as tokens, so the 12 and the px in var(--w)px never become one length. With --w: 12, a browser computes these:

Invalid at computed-value time is worse than invalid at parse time: the property falls back to its initial value and takes any earlier declaration of it with it. So text written against a hole is reported — a unit, a suffix, a # in front, on either side.

One value, read many times

Write the hole wherever you need the value. That is the whole answer for almost every block:

import type { Value } from "../css-system";

declare const accent: Value<"color">;

const card = @@(
  border-left: 4px solid {accent};
  background: {accent};
  color: {accent};
);

A hole belongs to the declaration it is written in, so this puts three custom properties on the element rather than one, all holding the same value — a rule is shared by every element that names it, so its variable cannot be named after anything but itself.

Whether that matters is a number, and the number is small. On a real block reading one value five times, measured: the style attribute is 102 B written directly, and 24 B if you declare a custom property once and read it. Seventy-eight bytes an element. On one card that is nothing; on a list of a hundred rows it is 7.8 KB of markup, and then it is worth a line:

declare const accent: string;

const row = @@(
  --accent: {accent};
  border-left: 4px solid var(--accent);
  background: var(--accent);
  color: var(--accent);
);

The three declarations below it have no hole at all now, so they are static classes that dedupe with every other block writing the same thing. Reach for this when a block repeats across many elements, not by default — the direct form is shorter to read and to write, and 78 bytes is not a reason.

The name is yours, and a custom property INHERITS. --accent is set on the element and is visible to everything inside it: measured, a descendant that reads var(--accent) and never sets one picks up the ancestor's value, a descendant that sets its own shadows it, and an element outside the subtree falls back. That is a feature when you mean it and a collision when you do not — a card setting --accent changes any descendant whose own block reads that name.

So pick a name you would be happy to see inherited, or one nobody else would write. The names the compiler generates for holes never have this problem: --r-<class>-0 is derived from the declaration itself, so two different declarations can never agree on one by accident.

A hole may not be empty

string | number, and nothing else. undefined and null are refused, so a value that might not be there needs a fallback written where it is used:

color: {tint};                   ✗  TS2345 — `null` is not a value
color: {tint ?? "inherit"};      ✓

An empty hole is not a declaration you can see. It is a var() with nothing behind it, which makes the whole declaration invalid at computed-value time — so the property falls back past every earlier declaration of it, including the one you spread in above.

On a server-rendered page it is worse in one direction: a hole that has a value on the server and none on the client is a divergence, and the page keeps showing the server's value. Write the empty case out and neither happens.

The value carrying a unit can be typed

A hole's value is string | number, and it has to be: 349 of 551 properties are composite. border-left is <line-width> || <line-style> || <color>, so 4px solid red in any order — a type narrow enough to refuse 4px sollid red would refuse red 4px solid, which is correct CSS.

So the type goes where the value is made, which is the better place anyway: the error lands on the line somebody wrote.

const border: CssDimension = `${weight}px`;      ✓
const border: CssDimension = `${weight}pddx`;    ✗  TS2322, on this line

No as const is needed — the annotation is the context — and it works in a getter, which is where a value like this usually comes from:

import type { CssDimension } from "@ramonda/css";

class Card {
  weight = 4;
  get border(): CssDimension {
    return `${this.weight}px`;
  }
}

The unit set is a parameter, so an app that has settled on one says so:

const gap: CssDimension<"px"> = `${n}rem`;       ✗  TS2322 — this app writes px

There is a union per family — CssLengthUnit, CssAngleUnit, CssTimeUnit, CssResolutionUnit and CssFrequencyUnit — so CssDimension<CssLengthUnit> is a length and refuses 12deg. All of them are generated from the same unit table the checker measures a typo against, so the two cannot disagree.

One looseness, on purpose: any call is admitted. calc(), min(), clamp() and var() can each produce any dimension and nothing in a type can read inside one, so calc(1rem + 2px) passes CssDimension<"px">. Refusing calls would make the type useless in the one place you reach for it.

In a project with a config, name the property instead

CssDimension says a length. When your project has a ramonda.css.ts, it can say whatever this property takes here — units narrowed, closed lists, variablesOnly and all:

import type { Value } from "../css-system";

declare const n: number;

const gap: Value<"gap"> = `${n}px`;

One property, one name, and the answer moves when the config does. Value<"color"> is what the hole example above is annotated with, for the same reason.

And Var<"color"> is any variable this project declares of that kind — which is what you want when a function chooses between them:

import { $ } from "../css-system";
import type { Var } from "../css-system";

declare const loud: boolean;

const pick = (): Var<"color"> => (loud ? $.color.accent : $.color.surface);

Comments

A block is CSS, so its comment is CSS's, and it is stripped from the emitted rule:

const card = @@(
  /* the dot is the level, and it is set from the theme */
  display: grid;
  gap: 8px;      /* matches the list beside it */
);

// is not a comment here, and it does not fail quietly. CSS has no line comment, so the characters go into the stylesheet as they stand:

.r-3f96…{// why
  color:red;gap:8px;}

and the CSS compiler refuses the whole file — measured, SyntaxError: Unexpected token Semicolon — naming nothing about the block, the file, or the line it came from. A build that fails somewhere else entirely, because of a comment. So it is reported before it gets there, as line-comment, on the // itself.

Inside a hole a comment is TypeScript's, because that is what a hole holds:

const card = @@(
  color: {/* the brand, not the accent */ "#ff0055"};
);

Next

• What is checked — every rule and what it catches.
• Composing — reusing a block, conditions, and which declaration wins.