`.a, .b::wat { color: red }` styles `.a`, where a browser drops the whole rule for its unknown pseudo-element

Start with src/compiler/selectors.ts, especially getSelectors and classNameSelector, to trace how selector lists handle custom pseudo-elements and pseudo-classes. Run the reproducer in src/__tests__/native/unknown-pseudo-selector-list.test.tsx; done means the two unknown-pseudo cases leave .a unstyled and produce a warning naming the unreadable selector, while .b::before still styles .a.

Before submitting a new issue

• I tested using the latest version of the library, as the bug might be already fixed.
• I tested using a supported version of react native.
• I checked for possible duplicate issues, with possible answers.

Bug summary

.a, .b::wat { color: red } makes .a red on native, and so does .a, .b:wat. In a browser neither rule applies anywhere: a pseudo-element or pseudo-class the browser does not know makes its selector invalid, and a selector list holding an invalid selector is invalid as a whole (Selectors 4 §3.7). Chromium 156, Firefox 157 and WebKit 27.2 leave .a unstyled for both, and style it for .a, .b::before, whose pseudo-element they know. compile().warnings() reports nothing.

The first two cases fail, .a receiving {"color":"#f00"}; .b::before passes.

getSelectors (src/compiler/selectors.ts) converts each selector of a rule's list on its own and skips one that classNameSelector returns null for, as it does for every pseudo-element and for every pseudo-class outside hover, active, focus, disabled and empty, so the rule keeps the list's other selectors. That is right for a name the browser knows, and wrong for one lightningcss parses as a custom pseudo-element or pseudo-class, whose list a browser drops whole.

Library version

main at 2421ea7 (0.0.0-nightly.2421ea7)

Environment info

System:
  OS: Windows 11 10.0.26300
  CPU: "(32) x64 AMD Ryzen 9 9950X3D 16-Core Processor          "
  Memory: 155.26 GB / 255.67 GB
Binaries:
  Node:
    version: 26.12.1
    path: C:\Program Files\nodejs\node.EXE
  Yarn:
    version: 4.9.2
    path: C:\Users\<user>\AppData\Roaming\npm\yarn.CMD
  npm:
    version: 12.2.1
    path: C:\Program Files\nodejs\npm.CMD
  Watchman: Not Found
SDKs:
  Android SDK:
    API Levels:
      - "33"
      - "35"
      - "35"
      - "35"
      - "36"
      - "37"
    Build Tools:
      - 35.0.0
      - 36.0.0
      - 36.1.0
      - 37.0.0
    System Images:
      - android-26 | Google APIs Intel x86_64 Atom
      - android-28 | Google APIs Intel x86_64 Atom
      - android-29 | Google APIs Intel x86_64 Atom
      - android-30 | Google APIs Intel x86_64 Atom
      - android-31 | Google APIs Intel x86_64 Atom
      - android-32 | Google APIs Intel x86_64 Atom
      - android-33 | Google APIs Intel x86_64 Atom
      - android-34 | Google APIs Intel x86_64 Atom
      - android-35 | Google APIs Intel x86_64 Atom
      - android-36.1 | Google Play Intel x86_64 Atom
      - android-36.1 | Pre-Release 16 KB Page Size Google Play Intel x86_64 Atom
      - android-37.0 | Google APIs Intel x86_64 Atom
    Android NDK: Not Found
  Windows SDK: Not Found
IDEs:
  Android Studio: AI-262.9437.185.2621.16467767
  Visual Studio: Not Found
Languages:
  Java:
    version: 17.0.20.1
    path: C:\Program Files\Microsoft\jdk-17.0.20.101-hotspot\bin\javac.EXE
  Ruby: Not Found
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.86.3
    wanted: 0.86.3
  react-native-windows: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: Not found
  newArchEnabled: Not found
iOS:
  hermesEnabled: Not found
  newArchEnabled: Not found

Steps to reproduce

1. Clone the reproducer branch, which is main at 2421ea7 with one file added, src/__tests__/native/unknown-pseudo-selector-list.test.tsx, and install:

   git clone --branch repro/521-unknown-pseudo-selector-list https://github.com/YevheniiKotyrlo/react-native-css
   cd react-native-css
   yarn install
   

2. Run yarn jest src/__tests__/native/unknown-pseudo-selector-list.test.tsx.

src/__tests__/native/unknown-pseudo-selector-list.test.tsx:

import { render, screen } from "@testing-library/react-native";
import { compile } from "react-native-css/compiler";
import { Text } from "react-native-css/components/Text";
import { registerCSS, testID } from "react-native-css/jest";

// Whether `.a` is red in Chromium 156, Firefox 157 and WebKit 27.2: browser-readings.json beside this file
const cases = [
  { selector: ".a, .b::wat", browsersStyleA: false },
  { selector: ".a, .b:wat", browsersStyleA: false },
  { selector: ".a, .b::before", browsersStyleA: true },
];

test.each(cases)(
  "$selector { color: red } styles .a as the browsers do: $browsersStyleA",
  ({ selector, browsersStyleA }) => {
    const css = `${selector} { color: red; }`;
    const compiled = compile(css);
    console.log(
      JSON.stringify({
        selector,
        rules: compiled.stylesheet().s?.map(([name]) => name),
        warnings: compiled.warnings(),
      }),
    );

    registerCSS(css);
    render(<Text testID={testID} className="a" />);

    const { style } = screen.getByTestId(testID).props;
    console.log(JSON.stringify({ selector, style }));

    expect(style !== undefined).toBe(browsersStyleA);
  },
);

Expected: neither .a, .b::wat nor .a, .b:wat styles .a, as in a browser, and a warning names the selector it could not read.

Actual: both style .a, and compile().warnings() reports nothing.

  console.log
    {"selector":".a, .b::wat","rules":["a"],"warnings":{}}
      at log (src/__tests__/native/unknown-pseudo-selector-list.test.tsx:18:13)
  console.log
    {"selector":".a, .b::wat","style":{"color":"#f00"}}
      at log (src/__tests__/native/unknown-pseudo-selector-list.test.tsx:30:13)
  console.log
    {"selector":".a, .b:wat","rules":["a"],"warnings":{}}
      at log (src/__tests__/native/unknown-pseudo-selector-list.test.tsx:18:13)
  console.log
    {"selector":".a, .b:wat","style":{"color":"#f00"}}
      at log (src/__tests__/native/unknown-pseudo-selector-list.test.tsx:30:13)
  console.log
    {"selector":".a, .b::before","rules":["a"],"warnings":{}}
      at log (src/__tests__/native/unknown-pseudo-selector-list.test.tsx:18:13)
  console.log
    {"selector":".a, .b::before","style":{"color":"#f00"}}
      at log (src/__tests__/native/unknown-pseudo-selector-list.test.tsx:30:13)
FAIL src/__tests__/native/unknown-pseudo-selector-list.test.tsx
  × .a, .b::wat { color: red } styles .a as the browsers do: false (33 ms)
  × .a, .b:wat { color: red } styles .a as the browsers do: false (6 ms)
  √ .a, .b::before { color: red } styles .a as the browsers do: true (2 ms)
  ● .a, .b::wat { color: red } styles .a as the browsers do: false
    expect(received).toBe(expected) // Object.is equality
    Expected: false
    Received: true
      30 |     console.log(JSON.stringify({ selector, style }));
      31 |
    > 32 |     expect(style !== undefined).toBe(browsersStyleA);
         |                                 ^
      33 |   },
      34 | );
      35 |
      at toBe (src/__tests__/native/unknown-pseudo-selector-list.test.tsx:32:33)
  ● .a, .b:wat { color: red } styles .a as the browsers do: false
    expect(received).toBe(expected) // Object.is equality
    Expected: false
    Received: true
      30 |     console.log(JSON.stringify({ selector, style }));
      31 |
    > 32 |     expect(style !== undefined).toBe(browsersStyleA);
         |                                 ^
      33 |   },
      34 | );
      35 |
      at toBe (src/__tests__/native/unknown-pseudo-selector-list.test.tsx:32:33)
Test Suites: 1 failed, 1 total
Tests:       2 failed, 1 passed, 3 total
Snapshots:   0 total
Time:        1.165 s, estimated 2 s
Ran all test suites matching /src\\__tests__\\native\\unknown-pseudo-selector-list.test.tsx/i.

Reproducible example repository

https://github.com/YevheniiKotyrlo/react-native-css/tree/repro/521-unknown-pseudo-selector-list