Debugging StyleX + Vite: The Mystery of 'Invalid Empty Selector'

Published: 17 hours ago (January 1, 2026 at 04:36 PM EST)

6 min read

Source: Dev.to

A methodical journey through debugging a CSS‑in‑JS race condition when the error message tells you nothing

TL;DR – If you’re getting Invalid empty selector errors from StyleX with Vite and you’re using imported constants as computed property keys like [breakpoints.tablet]: '1rem', that’s the problem. Replace them with inline strings such as "@media (max-width: 768px)": '1rem'. Below is why this happens and how we figured it out.

Common Error Messages (For Searchability)

If you landed here from a search, you might have seen one of these errors:

Error: Invalid empty selector

Invalid empty selector
unknown file:528:1
    at lightningTransform (node_modules/@stylexjs/unplugin/lib/es/index.mjs)

LightningCSS error: Invalid empty selector

@stylexjs/unplugin: Invalid empty selector

vite stylex Invalid empty selector

Or you might see @media var(--xxx) in your generated CSS, which is the actual cause of the error.

The Error That Told Us Nothing

It started with an error that gave us almost zero useful information:

Error: Invalid empty selector
unknown file:528:1
    at lightningTransform (node_modules/@stylexjs/unplugin/lib/es/index.mjs)
    at processCollectedRulesToCSS (node_modules/@stylexjs/unplugin/lib/es/index.mjs)
    at collectCss (node_modules/@stylexjs/unplugin/lib/es/index.mjs)

No source file, no line in our code, no indication of which style was broken—just “unknown file” and a line number in generated CSS that we couldn’t see.

This is the story of how we tracked down the root cause — and, more importantly, the debugging methodology that got us there.

Why This Pattern Should Work

Before diving into debugging, it’s important to understand: we weren’t doing anything wrong according to StyleX documentation.

StyleX provides stylex.defineConsts() for defining reusable constants like media queries. The official docs show exactly this pattern:

// This is the documented, recommended approach
import * as stylex from '@stylexjs/stylex';

export const breakpoints = stylex.defineConsts({
  tablet: "@media (max-width: 768px)",
  mobile: "@media (max-width: 640px)",
});

And using these constants as computed property keys is standard JavaScript:

import { breakpoints } from './breakpoints.stylex';

const styles = stylex.create({
  container: {
    padding: {
      default: '2rem',
      [breakpoints.tablet]: '1rem', // Standard JS computed property
    },
  },
});

This pattern works perfectly in:

Production builds
Webpack dev server
Next.js

But it breaks in Vite dev mode. The question was: why?

The Debugging Journey

Step 1 – The Obvious First Attempts

# Clear all caches
rm -rf node_modules/.vite
rm -rf node_modules/.cache
npm run dev

# Nuclear option – reinstall everything
rm -rf node_modules
npm install
npm run dev

Both attempts left the error untouched, so caching wasn’t the issue.

Step 2 – Understanding the Error Source

The stack trace pointed to lightningTransform in @stylexjs/unplugin. LightningCSS is the CSS parser/transformer that StyleX uses. The error “Invalid empty selector” meant LightningCSS was receiving malformed CSS.

Key insight: The error wasn’t in our source code — it was in the generated CSS, which we couldn’t see.

Step 3 – Instrumenting the Build Pipeline

Because we couldn’t view the intermediate CSS, we added a debug hook directly inside node_modules/@stylexjs/unplugin/lib/es/index.mjs:

function processCollectedRulesToCSS(rules, options) {
  if (!rules || rules.length === 0) return '';

  const collectedCSS = stylexBabelPlugin.processStylexRules(rules, {
    useCSSLayers: options.useCSSLayers ?? false,
    classNamePrefix: options.classNamePrefix ?? 'x',
  });

  // DEBUG: always write the CSS so we can see what’s being generated
  const fs = require('fs');
  const lines = collectedCSS.split('\n');
  console.log('[StyleX DEBUG] CSS lines:', lines.length);
  fs.writeFileSync(`stylex-debug-${lines.length}.css`, collectedCSS);

  let code;
  try {
    const result = lightningTransform({
      filename: 'styles.css',
      code: Buffer.from(collectedCSS),
      minify: options.minify ?? false,
    });
    code = result.code;
  } catch (error) {
    // CRITICAL: capture the CSS that caused the failure
    fs.writeFileSync('stylex-debug-FAILED.css', collectedCSS);
    console.log('[StyleX DEBUG] FAILED – check stylex-debug-FAILED.css');
    throw error;
  }

  return code.toString();
}

Why this matters: When debugging build tools, you often can’t see intermediate artifacts. Adding instrumentation to capture them is essential.

Step 4 – The First Clue

Running the dev server after the instrumentation produced stylex-debug-FAILED.css. Opening it revealed:

@media var(--xgageza) {
  .x1abc123 {
    padding-left: 1rem;
  }
}

@media var(--xgageza) is not valid CSS – it’s the exact thing LightningCSS complained about.

Step 5 – Tracing the Origin of `var(--xgageza)`

Searching the generated CSS showed that every media query generated by StyleX was being replaced with a CSS variable reference. The culprit turned out to be the way Vite’s dev server handled computed property keys that reference imported constants. The constants were being evaluated after StyleX’s Babel plugin had already collected the rules, resulting in a placeholder variable (var(--xgageza)) that never got resolved.

Step 6 – Confirming the Hypothesis

We created a minimal repro:

// breakpoints.stylex
import * as stylex from '@stylexjs/stylex';
export const bp = stylex.defineConsts({
  tablet: '@media (max-width: 768px)',
});

// component.jsx
import { bp } from './breakpoints.stylex';
export const styles = stylex.create({
  box: {
    [bp.tablet]: { padding: '1rem' },
  },
});

Webpack → builds fine.
Vite (dev) → throws Invalid empty selector.

Switching to an inline string eliminated the error:

const styles = stylex.create({
  box: {
    '@media (max-width: 768px)': { padding: '1rem' },
  },
});

Thus the problem is the runtime import of the constant in Vite’s dev mode.

Step 7 – Work‑around / Fix

Replace computed‑property usage with literal strings, or move the constants into a plain object (no stylex.defineConsts) and reference them directly:

// breakpoints.js – plain object, no StyleX API
export const breakpoints = {
  tablet: '@media (max-width: 768px)',
  mobile: '@media (max-width: 640px)',
};

// component.jsx
import { breakpoints } from './breakpoints';
export const styles = stylex.create({
  container: {
    [breakpoints.tablet]: { padding: '1rem' },
  },
});

Alternatively, upgrade to a newer version of @stylexjs/unplugin (≥ 0.5.2) where the issue is fixed, or configure Vite to pre‑bundle the constants with optimizeDeps.

Takeaways & Debugging Methodology

Start with the obvious – clear caches, reinstall, restart.
Read the stack trace – locate the tool that throws the error (LightningCSS).
Capture intermediate artifacts – instrument the build pipeline to write out generated CSS.
Search for patterns – the malformed @media var(--…) pointed to a transformation bug.
Isolate the minimal repro – strip down to the smallest example that reproduces the error.
Test hypotheses – replace the constant with a literal string; the error disappears.
Apply a fix or workaround – either avoid the problematic pattern in Vite dev mode or upgrade the plugin.

By following these steps, you can turn an opaque “Invalid empty selector” into a concrete, reproducible bug and resolve it quickly. Happy debugging!

CSS Variable Reference, Not a Media Query!

The CSS should have been:

@media (max-width: 768px) {
  .x1abc123 {
    padding-left: 1rem;
  }
}

The Smoking Gun

Something was generating var(--xgageza) where @media (max-width: 768px) should be.

Step 5 – The Wrong Hypothesis

Our first hypothesis: “Maybe stylex.defineConsts() is broken. Let’s try plain JavaScript objects instead.”

// Changed from stylex.defineConsts() to plain object
export const breakpoints = {
  tablet: "@media (max-width: 768px)",
  mobile: "@media (max-width: 640px)",
};

We cleared caches, restarted… still broken.

Step 6 – Following the Evidence

We went back to the failed CSS file and searched for all instances of var(--. There were many, and they all followed a pattern — they appeared where media‑query strings should be.

Then we looked at how these values were being used, not just how they were defined:

// In roles.stylex.js
const styles = stylex.create({
  heading: {
    paddingBlock: {
      default: '1.5rem',
      [breakpoints.tablet]: '1.25rem', //  **Have you encountered similar CSS‑in‑JS race conditions?**  
      // > What’s your approach to debugging build‑tool issues? Share your stories in the comments.
    },
  },
});

Keywords (for search engines)

StyleX Invalid empty selector
StyleX Vite error
@stylexjs/unplugin error
LightningCSS Invalid empty selector
StyleX stylex.create media query error
StyleX breakpoints not working
StyleX defineConsts Vite
StyleX computed property key
Vite CSS‑in‑JS race condition
StyleX var(--…) in media query
@media var CSS error
StyleX unplugin lightningTransform error
StyleX Vite dev‑mode crash
processCollectedRulesToCSS error

Debugging StyleX + Vite: The Mystery of 'Invalid Empty Selector'

Common Error Messages (For Searchability)

The Error That Told Us Nothing

Why This Pattern Should Work

The Debugging Journey

Step 1 – The Obvious First Attempts

Step 2 – Understanding the Error Source

Step 3 – Instrumenting the Build Pipeline

Step 4 – The First Clue

Step 5 – Tracing the Origin of `var(--xgageza)`

Step 6 – Confirming the Hypothesis

Step 7 – Work‑around / Fix

Takeaways & Debugging Methodology

CSS Variable Reference, Not a Media Query!

The Smoking Gun

Step 5 – The Wrong Hypothesis

Step 6 – Following the Evidence

Tags

Keywords (for search engines)

Related posts

StyleX + ESLint: When Best Practices Fight Each Other

Building a Modern POS System That Doesn't Feel Like It's From 2005

🪝Finally Started the Most Awaited Section: Custom Hooks, Refs & More State

How I Built a Client-Side Image Resizer (Lightning Fast) using Vanilla JS

Common Error Messages (For Searchability)

The Error That Told Us Nothing

Why This Pattern Should Work

The Debugging Journey

Step 1 – The Obvious First Attempts

Step 2 – Understanding the Error Source

Step 3 – Instrumenting the Build Pipeline

Step 4 – The First Clue

Step 5 – Tracing the Origin of var(--xgageza)

Step 6 – Confirming the Hypothesis

Step 7 – Work‑around / Fix

Takeaways & Debugging Methodology

CSS Variable Reference, Not a Media Query!

The Smoking Gun

Step 5 – The Wrong Hypothesis

Step 6 – Following the Evidence

Tags

Keywords (for search engines)

Related posts

StyleX + ESLint: When Best Practices Fight Each Other

Building a Modern POS System That Doesn't Feel Like It's From 2005

🪝Finally Started the Most Awaited Section: Custom Hooks, Refs & More State

How I Built a Client-Side Image Resizer (Lightning Fast) using Vanilla JS

Step 1 – The Obvious First Attempts

Step 2 – Understanding the Error Source

Step 3 – Instrumenting the Build Pipeline

Step 4 – The First Clue

Step 5 – Tracing the Origin of `var(--xgageza)`

Step 6 – Confirming the Hypothesis

Step 7 – Work‑around / Fix

Step 5 – The Wrong Hypothesis

Step 6 – Following the Evidence