EPUB LSP

A Language Server Protocol implementation for EPUB source files. Validates XHTML content documents, OPF package files, navigation documents, and CSS with rules based on epubcheck and ace. Pure Go, no Java or Node.js dependencies.

Installation

Download Binary

Download prebuilt binaries from GitHub Releases.

Available for Linux, macOS, and Windows (amd64/arm64).

Install from Source

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

Build from Source

git clone https://github.com/toba/epub-lsp.git
cd epub-lsp
go build -o epub-lsp ./cmd/epub-lsp
go test ./...

Editor Integration

epub-lsp communicates over stdin/stdout using JSON-RPC per the LSP specification. Point your editor's LSP client at the epub-lsp binary for .opf, .xhtml, .html, and .css files.

A Zed extension is available at gubby.

Supported File Types

Extension	Type	Detection
.opf	OPF package document	Extension
.xhtml	XHTML content document	Extension
.html	HTML content document	Extension
.css	CSS stylesheet	Extension
.ncx	NCX navigation (EPUB 2)	Extension

Navigation documents (.xhtml/.html containing epub:type="toc") are detected via content sniffing and receive additional nav-specific validation.

Validators

OPF Package Document

• Required metadata: dc:identifier, dc:title, dc:language
• unique-identifier must reference a valid dc:identifier/@id
• Manifest integrity: unique IDs, valid media-types, no duplicate hrefs
• Spine itemrefs must reference existing manifest items

XHTML Content Document

• XHTML namespace (xmlns="http://www.w3.org/1999/xhtml") required
• xml:lang and lang consistency
• <img> elements must have alt attribute

Navigation Document

• <nav epub:type="toc"> required with <ol> child
• No remote links allowed in navigation
• Optional page-list and landmarks detection
• TOC link order vs spine order consistency

CSS Stylesheet

• Forbidden properties: direction, unicode-bidi
• Position warnings: fixed, absolute
• @font-face format validation (woff, woff2, opentype, truetype)
• UTF-8 encoding check
• Unclosed brace detection

Cross-File Resource Validation

• Manifest items reference files that exist in the workspace
• Resources referenced in content (<img>, <link>, <audio>, <video>, <source>) exist in the OPF manifest

Accessibility (based on DAISY Ace rules)

• Metadata: schema:accessMode, schema:accessibilityFeature, schema:accessibilityHazard, schema:accessibilitySummary, schema:accessModeSufficient with value validation and contradictory hazard detection
• OPF: dc:title and dc:language presence
• Page navigation: printPageNumbers requires page-list nav and pagebreak markers; page-list requires dc:source; page-list references validated against content IDs
• Structure: epub:type to ARIA role mapping, pagebreak labels, heading level ordering, table captions, form input labels

Architecture

cmd/epub-lsp/           LSP server (stdin/stdout JSON-RPC)
  lsp/                  Protocol types, message framing, handlers
internal/epub/          Core types (Diagnostic, FileType, Position)
  parser/               XML and CSS parsers with offset tracking
  testutil/             Shared test helpers
  validator/            Registry and Validator interface
    opf/                OPF package validation + OPF parsing
    xhtml/              XHTML namespace and structure checks
    nav/                Navigation document validation
    css/                CSS property and syntax checks
    resource/           Cross-file manifest and content reference checks
    accessibility/      Accessibility metadata, structure, and page checks

Validators register with a central Registry and are dispatched by file type. Files within a workspace are validated concurrently. Cross-file context (manifest items, spine order, file contents) is passed via WorkspaceContext.

License

MIT License - see LICENSE for details.