If you composes: a class that doesn't exist in the same file, lightningcss silently accepts it. The exports object
points to a hashed name that's never emitted as a CSS rule, so the class lands on the DOM but has no matching
styles. No error, no warning.

.historyChevron { color: blue; }

.historyChevronOpen {
  composes: historychevron; /* typo, real class is .historyChevron */
  transform: rotate(180deg);
}

cssModules: true, builds fine. color: blue never inherits.

postcss-modules-scope errors on this with referenced class name "historychevron" in composes not found
(source).

In src/css_modules.rs handle_composes, the same-file branch hashes name.0.as_ref() into a reference with no
lookup against exports_by_source_index. The cross-file branch does look up but silently continues on miss.