go-css-lsp

A Language Server Protocol implementation for pure, modern CSS, written in Go. Designed as a lightweight, fast alternative to vscode-css-languageservice.

Features

Category	Capabilities
Diagnostics	Unknown properties, duplicates, unknown at-rules, experimental property warnings, deprecated property warnings, empty rulesets, !important hints, vendor prefix hints, zero-with-unit hints, parse errors
Hover	Property documentation with MDN references, experimental status indicators
Completion	Properties, values, at-rules, pseudo-classes, pseudo-elements, HTML elements, color functions; experimental features tagged
Colors	Color picker for hex, named colors, rgb(), hsl(), hwb(), lab(), lch(), oklab(), oklch(); convert between formats
Navigation	Go to definition, find references, document symbols, document highlights
Editing	Rename CSS custom properties, code actions (quick fixes), formatting (expanded/compact/preserve/detect modes), selection ranges
Structure	Folding ranges, document links (@import, url())
Workspace	Cross-file CSS custom property indexing

Editor Support

Used by the Gossamer Zed extension. Compatible with any editor that supports LSP.

Installation

Download a binary from Releases, or build from source:

go install github.com/toba/go-css-lsp/cmd/go-css-lsp@latest

Building

go build ./cmd/go-css-lsp

Testing

go test ./...

Formatting Modes

The formatter supports four modes, configured via initializationOptions:

Mode	Behavior
expanded (default)	One declaration per line
compact	Single-line rulesets when they fit within printWidth
preserve	Keeps original single/multi-line layout, normalizes whitespace
detect	Infers intent from source: if the first property is on the same line as { and the result fits printWidth, single-line; otherwise expanded

Rulesets containing nested rules always use expanded format regardless of mode.

{
  "initializationOptions": {
    "formatMode": "compact",
    "printWidth": 80,
    "experimentalFeatures": "warning",
    "deprecatedFeatures": "warning"
  }
}

Setting	Type	Default	Description
formatMode	string	"expanded"	"expanded", "compact", "preserve", or "detect"
printWidth	int	80	Max line width for compact/detect modes
experimentalFeatures	string	"warning"	How to handle experimental CSS features: "ignore", "warning", or "error"
deprecatedFeatures	string	"warning"	How to handle deprecated (obsolete) CSS features: "ignore", "warning", or "error"

Experimental Features

Nonstandard CSS properties are filtered out entirely and produce "unknown property" warnings. Experimental properties (e.g. field-sizing) are recognized but flagged based on the experimentalFeatures setting:

Value	Diagnostics	Completions
"ignore"	None	No tagging
"warning" (default)	Warning severity	Tagged (experimental)
"error"	Error severity	Tagged (experimental)

Deprecated Features

Deprecated (obsolete) CSS properties (e.g. clip) are recognized and flagged based on the deprecatedFeatures setting:

Value	Diagnostics	Completions
"ignore"	None	No tagging
"warning" (default)	Warning severity	Tagged (deprecated), strikethrough
"error"	Error severity	Tagged (deprecated), strikethrough

Credits

This project was built using the following as references for feature design and architecture:

• microsoft/vscode-css-languageservice (MIT) — CSS language intelligence and LSP feature set. The primary reference for what capabilities a CSS language server should provide.
• lmn451/css-variables-zed (GPL-3.0) — workspace-wide CSS variable indexing approach.

License

MIT — see LICENSE for details.