As the library is mobile-first by default, any @media expressions take higher priority over the default (mobile) values.

.composition {
  padding: 10px;

  @media (max-width: 768px) { /* prop with a "down" behavior */
    padding: 20px; /* will be "20px" on mobile as well */
  }
}

Perhaps, listing down behavior is not compliant with the mobile-first approach. I see two ways of solving this issue:

1. Increase specificity of default (mobile) properties. This would most likely mean some explicit selector, or logic, which I would be against.
2. Remove defaultBehavior option and make Atomic layout mobile-first always. Alternatively, remove the "down" value of this option, leaving "up" and "only" (depends if "only" makes sense in case of responsive props application).

It would be nice to have this fixed, yet it's likely that this API would be deprecated in the future (#165). There must be a way to resolve this CSS specificity issue without altering the library's API.

CSS-wise, there is no API to separate such two rules specificity, as the second one's will always be higher by the fact of later declaration, and the application surface. We are speaking about the scenario when the screen satisfies the media query, but the root-level property must be applied (see the beginning of the thread).

.element {
  color: red;
 
  @media (max-width: 700px) {
    // When should I stop being blue and become red?
    // There is no programatic way to tell.
    color: blue;
  }
}

That's not a CSS issue, but the issue with how a media query is constructed for the responsive props with the "down" suffix. I think such responsive props declarations should:

1. Try to find the closest preceding sibling property;
2. If found, prepend its breakpoint to the composed media query.

Example

Preceding sibling prop present

<Composition
  templateCols="200px auto"
  templateColsSmDown="400px 1fr"
/>

When parsing templateColsSmDown:

1. Takes the props with the same name (templateCols) and tries to find the first sibling iterating down the list of props.
2. Finds templateCols="200px auto" ("xs" breakpoint).
3. Prepends breakpoints.xs.maxWidth as the min-width parameter of the media query:

/*     "xs" prefix            "sm" closed breakpoint */
@media (min-width: 500px) and (max-width: 576px) {
  grid-template-columns: 400px 1fr;
}

Preceding sibling prop missing

<Composition
  padding={10}
  templateColsSmDown="400px 1fr"
/>

When parsing templateColsSmDown:

1. Takes the props with the same name (templateCols) and tries to find the first sibling iterating down the list of props.
2. Finds nothing, treats templateColsSmDown as if it should apply all the way down.
3. Generates the following media query:

@media (max-width: 576px) {
  grid-template-columns: 400px 1fr;
}

Technical aspect

Such changes may require to refactor the way applyStyles works. If the sibling props are grouped first, then it would be possible to analyze them to determine whether responsive props with "down" behavior should be closed.

Due to the amount of possible variations in responsive props declaration, I suggest the solution to this problem would be as simple as possible:

• Whenever a "down" responsive prop is met, it iterates down (or straight jumps to the first record, if it's not the first one already) in its records and when it finds a record with no explicit breakpoint (treating this as the default breakpoint), and encloses the media query of "down" responsive prop according to that found default breakpoint record.

All the other use cases are rather abstract and not likely to appear in real world usage. I wouldn't invest time and library's size into figuring out how to treat them until a real necessity arises.