# Syntax highlighting, and what it costs

Established 2026-08-02: module footprint and binary sizes measured by building a probe module, the
alternatives surveyed by search. Numbers are for chroma v2.27.0 on darwin/arm64.

## There is one mature pure-Go option

chroma. Every "alternative to chroma" a search returns — Prism, highlight.js, Rainbow — is JavaScript, and
therefore unavailable to a theme that ships no script. `zyedidia/highlight` exists but needs syntax files
supplied per language and covers far less.

## What chroma costs

- **Two modules**: `github.com/alecthomas/chroma/v2` and `github.com/dlclark/regexp2/v2`. Nothing else.
- **~5MB of binary.** A probe importing lexers, formatters and styles built at 7.4MB against a 2.4MB
  baseline; khosra itself went from ~15MB to ~19MB.
- chroma is larger than khosra.

## What it gives beyond colour

`html.WithLineNumbers`, `html.BaseLineNumber(n)` for a custom starting number, and
`html.HighlightLines([][2]int)`. With `html.WithClasses(true)` it emits token classes and no inline colour,
so the palette stays in a stylesheet the theme owns.

## Custom lexers need no rebuild

`chroma.Unmarshal(data []byte) (*RegexLexer, error)` reads a lexer from XML — the same format chroma's own
lexers ship in, under `lexers/embedded/*.xml`. With `chroma.NewLexerRegistry()` a site root could carry its
own language definition and register it at startup. Not built; the door is open by construction.

## What chroma does not give

A filename or title on a block, and content read from a file. Those are the engine's, whatever highlights.
So a design that parses the fence's info string works identically with chroma, without it, or after it.