Gossamer

A Zed editor extension providing modern, pure CSS language intelligence, powered by go-css-lsp.

Features

• Diagnostics — unknown properties, duplicates, experimental property warnings, deprecated property warnings, empty rulesets, parse errors, vendor prefix and !important hints
• Hover — property documentation with experimental status indicators
• Completion — properties, values, at-rules, pseudo-classes, pseudo-elements, color functions; experimental features tagged
• Document Colors — color picker for hex, named colors, rgb(), hsl(), hwb(), lab(), lch(), oklab(), oklch()
• Color Presentations — convert between color formats
• Document Symbols — outline view
• Go to Definition — CSS custom properties
• Find References — variables and selectors
• Rename — CSS custom properties
• Document Highlights — same-symbol highlighting
• Code Actions — quick fixes, source.fixAll for auto-fix on save
• Folding Ranges — code folding
• Document Links — clickable URLs
• Formatting — CSS pretty-printer with expanded, compact, preserve, and detect modes (supports format on save)
• Selection Ranges — expand/shrink selection

Installation

1. Open Zed
2. Go to Extensions (Cmd+Shift+X)
3. Search for "Gossamer"
4. Click Install

The extension automatically downloads the go-css-lsp binary for your platform.

As a Dev Extension:

1. Clone this repository
2. In Zed, open the command palette (Cmd+Shift+P)
3. Run "zed: install dev extension"
4. Select this directory

Format on Save:

Add to your Zed settings.json:

{
  "languages": {
    "CSS": {
      "format_on_save": "on",
      "formatter": {
        "language_server": {
          "name": "Gossamer"
        }
      }
    }
  }
}

Auto-fix on Save:

Add to your Zed settings.json to automatically fix diagnostics (e.g. remove unnecessary units like 0px → 0) on save:

{
  "languages": {
    "CSS": {
      "code_actions_on_format": {
        "source.fixAll": true
      }
    }
  }
}

Configuration

Configure via initialization_options in your Zed settings.json:

{
  "lsp": {
    "Gossamer": {
      "initialization_options": {
        "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"

Formatting Modes

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.

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

Building

cargo build --target wasm32-wasip1

License

MIT License — see LICENSE for details.