Which declaration wins

Two declarations of the same property, and both apply. Plain CSS answers with whichever was written last. A block answers with a rule, and it is the one every atomic CSS framework arrived at: the more specific case wins, whatever order you wrote it in.

The four things that decide it

In this order:

1  a condition beats no condition          padding: 8px          then  @media … { padding: 40px }
2  a longhand beats its shorthand          padding: 8px          then  padding-left: 40px
3  a narrower max-width beats a wider one  max-width: 64rem      then  max-width: 40rem
4  a wider min-width beats a narrower one  min-width: 40rem      then  min-width: 64rem
5  a standard property beats its prefix    -webkit-box-shadow    then  box-shadow

So this does what you meant, and the order you wrote the two breakpoints in does not matter:

const card = <div css={@@(
  padding: 8px;
  @media (min-width: 40rem) { padding: 16px; }
  @media (min-width: 64rem) { padding: 24px; }
)}>…</div>;

At 70rem the element gets 24px. Not because that line is last, but because 64rem is the narrower case — write the three in any order and the answer is the same.

Write them out of order and you are told. A declaration that cannot override the one above it is refused rather than quietly ignored:

padding is written to override @media (min-width: 64rem) above it, and it will not — the stylesheet emits the rule that applies to a wider viewport first, so the earlier one wins wherever both apply. Write it above, or put it under the same condition.

A prefix is a fallback, so it goes first

-webkit-box-shadow and box-shadow are the same property to the engine, so one of them has to be written first and the other wins. The sheet always puts the prefixed one first, which is what a prefixed fallback means and is the order you would write by hand:

const card = @@(
  -webkit-box-shadow: 0 0 1px #0003;
  box-shadow: 0 0 9px #0003;
);

Written the other way round it is refused, because the sheet cannot give that order:

-webkit-box-shadow is written to override box-shadow above it, and it will not — the stylesheet emits a vendor prefix before the standard property it is another name for, so the earlier one wins wherever both apply.

You rarely need one. This stylesheet is built on @layer, and every engine that has cascade layers already has the unprefixed transform, box-shadow, user-select and appearance.

Two conditions that can both be true

A stylesheet has one position for each rule, and two conditions that may hold at the same time each want a different one. There is no order to give them, so it is refused:

const card = @@(
  @supports (display: grid) { color: red; }
  @supports (display: flex) { color: blue; }
);

@supports (display: grid) and @supports (display: flex) can both hold at once, and the stylesheet cannot be ordered for both — it has one position for each rule, and whichever the build reads first would win.

Conditions that exclude each other are fine, and they are most of what anybody writes — a colour scheme, an orientation, a medium. No element is ever matched by both, so nothing has to be ordered:

const card = @@(
  @media (prefers-color-scheme: light) { color: #111; }
  @media (prefers-color-scheme: dark) { color: #eee; }
);

A condition your code decides

if ({ … }) { … } is not a stylesheet condition at all — it is a group merged when the expression holds, so the answer comes from the merge rather than from the sheet. Later wins, which is the order you wrote:

const card = (compact: boolean, loud: boolean) => @@(
  if ({compact}) { color: red; }
  if ({loud}) { color: blue; }
);

With both true the element is blue. Not sometimes — a block is one merge of maps in source order, and the element ends up carrying one class for color, chosen before anything reaches the page.

That is why these need no rule and no ordering band: the two above never both reach the stylesheet, so there is no position for them to fight over. Two CSS conditions are the opposite case — both rules exist in the sheet and something has to decide between them — which is the section above.

A mode is not a size

prefers-color-scheme and @media print are weaker than a breakpoint; @supports, orientation, prefers-contrast and forced-colors are stronger. So a theme goes above the breakpoints that refine it:

const card = <div css={@@(
  @media (prefers-color-scheme: dark) { color: white; }
  @media (min-width: 64rem) { color: black; }
)}>…</div>;

The full order, weakest first:

prefers-reduced-motion  →  prefers-color-scheme  →  print  →  BREAKPOINTS
   →  @supports  →  orientation  →  prefers-contrast  →  forced-colors

Breakpoints sit in the middle, so a breakpoint beats a dark-mode rule and forced-colors beats a breakpoint. That is Tailwind's order, and the reason to keep it is which mistake stays quiet: theming usually lives in the block you reuse, and the block reusing it usually adjusts at a breakpoint.

A condition this list does not name — @media (min-height: …), @media (hover: hover), a width in a unit that cannot be turned into a number — comes last. Two of those against each other is the one case with no rule: nothing tells them apart, so the one you wrote last wins. Put them under one condition if it matters.

Reusing a block does not change any of it

...{base} merges another block's declarations. Two declarations of one property under different conditions are two different things set, so both survive the merge and the list above decides:

const base = @@( @media (prefers-color-scheme: dark) { color: white; } );

const card = <div css={@@(
  ...{base};
  @media (min-width: 40rem) { color: blue; }
)}>…</div>;

At 40rem and up the element is blue, in dark mode too — the breakpoint is further down the list.

Write it the other way round and you are told, at run time in development:

[@ramonda/css] `color` is composed later under `@media (prefers-color-scheme: dark)` than under
`@media (min-width: 40rem)`, and it will not override it — the stylesheet emits the stronger
condition last, so the earlier one wins wherever both apply. Put the two under one condition, or
compose them the other way round.

Inside one block that is a build error instead, on the line you wrote. Across a reuse it can only be a run-time warning: what is in ...{base} is a value, and the compiler does not know it. It is said once, and it is not in a production build at all.

The one place the stylesheet decides instead of you

Everything above is decided where you wrote it. There is one exception, it is reported rather than silent, and it is worth understanding once.

A stylesheet has one order, and a rule in it is shared by every element that names it — so it cannot follow any single block's order. It emits unconditional rules before conditional ones, which is what makes the ordinary shape right:

const card = @@(
  padding: 8px;
  @media (min-width: 40rem) { padding: 24px; }   /* wins on a wide screen, as you would expect */
);

And it is why the opposite cannot compile: a block that puts the conditional declaration first is asking the sheet for an order it has no way to give, so it is refused with the sentence above rather than emitted and quietly wrong.

If you read the output

You will see numbered layers inside ramonda:

@layer ramonda.u00, …, ramonda.u17, ramonda.c;
@layer ramonda {
  @layer u09 { .r-p-8px { padding: 8px; } }
  @layer c { … @media (min-width: 40rem) { … } … }
}

That is the list above, made into cascade layers — a layer's place is decided by that statement rather than by where its rules sit, which is what keeps the order the same however the files load.

You never write against these names. ramonda is the name to put in your own @layer statement, and it orders everything inside it — see composing.

Composing — merging blocks, conditions, and your own stylesheet.
What is checked — every rule, including the two above.