Core

Rheo has a plugin architecture. Its core engine embeds the Typst compiler and provides shared infrastructure such as spine resolution, asset management, relative link rewriting, and a development server with live reloading. On top of this core, each output format is implemented as a format plugin.

Format plugins

Typst natively supports PDF and, experimentally, HTML. Rheo defines a FormatPlugin trait that can be implemented to produce additional formats beyond those that Typst natively supports. For example, Rheo supports EPUB as a format plugin, enabling the production of a standards-compliant EPUB archive from source Typst.

Each format plugin declares:

• A compile function specifying how build output files should be produced from the source spine.
• Any format-specific assets it needs – such as the HTML plugin’s CSS stylesheet and JavaScript entrypoint.
• Optional template files and configuration that are scaffolded when you run rheo init.

The core engine discovers source files, resolves the spine for each format, copies assets, and invokes each plugin’s compilation in turn.

An open architecture

Our hope is that this plugin architecture will inspire innovation at the format level. Typst is a remarkably expressive document language, and we believe its utility extends well beyond the formats it natively targets.

We have built Rheo as infrastructure for augmented reading research. Reading augmentations – features like hyperlink previews, contextual definitions, interactive visualizations, and explorable explanations – can be implemented either as Typst library functions (at the language level) or as Rheo format plugins (at the compiler level). The Rheo packages repository gives various examples of how custom Typst functions coupled with format-specific assets (such as JS and CSS in the case of HTML) can produce augmentations, both in the browser and in the other formats Rheo supports.