fix(css/parser): bom parsing

[ematipico]

Summary

Parsing of BOM was incorrect. The identifier lexing was including BOM as identifeir, breaking stuff.

I moved the BOM lexing before the identifier lexing.

Had help with AI.

Test Plan

Now we match prettier's formatting, so a snapshot was deleted

Docs

N/A

[changeset-bot]

🦋 Changeset detected

Latest commit: 1c692fd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 13 packages

Name	Type
@biomejs/biome	Patch
@biomejs/cli-win32-x64	Patch
@biomejs/cli-win32-arm64	Patch
@biomejs/cli-darwin-x64	Patch
@biomejs/cli-darwin-arm64	Patch
@biomejs/cli-linux-x64	Patch
@biomejs/cli-linux-arm64	Patch
@biomejs/cli-linux-x64-musl	Patch
@biomejs/cli-linux-arm64-musl	Patch
@biomejs/wasm-web	Patch
@biomejs/wasm-bundler	Patch
@biomejs/wasm-nodejs	Patch
@biomejs/backend-jsonrpc	Patch

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📥 Commits

Reviewing files that changed from the base of the PR and between f7bf12b and 1c692fd.

⛔ Files ignored due to path filters (1)

• crates/biome_css_formatter/tests/specs/prettier/css/bom/bom.css.snap is excluded by !**/*.snap and included by **

📒 Files selected for processing (2)

• .changeset/fix-css-bom-formatting.md
• crates/biome_css_parser/src/lexer/mod.rs

---

Walkthrough

Fixes a CSS formatter issue where Byte Order Mark (BOM) at the start of a file caused selector collapse. The parser lexer now prioritises BOM detection when at file position zero, preserving line breaks between comments and selectors in BOM-prefixed CSS files to align with Prettier's behaviour. Changes consolidate BOM logic into the initial Unicode identifier token handling branch and include a minor token mapping adjustment.

Suggested labels

A-Parser, L-CSS

Suggested reviewers

• dyc3
• denbezrukov

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: fixing BOM (byte order mark) parsing in the CSS parser, which is the core issue addressed across all modified files.
Description check	✅ Passed	The description clearly relates to the changeset, explaining the BOM parsing problem, the solution (moving BOM lexing before identifier lexing), and acknowledging AI assistance as required.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/css-bom-parser

---

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[codspeed-hq]

Merging this PR will not alter performance

✅ 29 untouched benchmarks
⏩ 187 skipped benchmarks¹

---

_{Comparing fix/css-bom-parser (1c692fd) with main (2d0b8e6)}

Footnotes

1. 187 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[denbezrukov]

🔥🔥🔥

[denbezrukov]

I'm wondering how you found this?

[ematipico]

I'm wondering how you found this?

I checked the Prettier tests we have, and noticed we have snapshot for the BOM formatting