Optional features and extensions

Carve documents remain readable when an optional feature is unavailable. The syntax still parses; only the richer behavior may be missing. Use this page to choose features for an application and understand their fallback.

For the API rules used to implement an extension, read the extension contract. For a complete worked example, see Write an extension.

What is available

Availability	What it means	Default
Core	Every conforming Carve processor supports it.	On
Standard optional	The reference implementations provide it, but applications enable it explicitly.	Off
Application extension	A particular implementation or host supplies it. Check that application.	Off

These are availability groups, not quality levels. “Optional” means that a document has a useful fallback when the feature is not enabled.

Frequently used optional features

Feature	Availability	What it adds	Without the feature
Citations	Standard optional	[@key] and locators	Citation source remains visible
List tables	Standard optional	Block content, row spans, and column spans in table cells	The containing list remains readable
Details	Standard optional	Native <details> and <summary> disclosure	A normal details container
Spoiler	Standard optional	Hidden content with an accessible reveal control	A visible generic span or container
Tabs	Standard optional	A set of named panels	Panels remain ordinary document content
Code callouts	Standard optional	Numbered explanations attached to code	Code and explanation list remain
Semantic spans	Standard optional	samp, var, cite, and dfn elements	A span carrying the semantic attribute

The feature availability page includes core syntax and features that are available by default with an opt-out.

Features supplied by applications

These depend on a renderer, data source, or user-interface environment and may not exist in every implementation:

• Mermaid and other diagram renderers
• bibliography formatting from CSL-JSON data as an option on citations
• glossary and index generation
• table-of-contents placement
• heading numbers and permalinks
• tabbed code groups
• external-link policies
• wikilinks and references to headings in the current document
• color swatches and other presentation helpers
• sanitized SVG image fences

Check the documentation for your chosen implementation before designing a workflow around one of these features.

Enable a feature

Extensions are registered on a processor or renderer. The exact constructor names differ by language, but the workflow is the same:

1. Create the extension or choose a supplied one.
2. Register it when creating the processor.
3. Render a representative document and inspect its fallback with the extension disabled.
4. For interactive output, also test the static-output fallback.

Implementation-specific setup is linked from Implementations and tooling.

Unknown extension names

The inline form :name[text] and block form ::: name are core syntax. If no handler recognizes name, Carve uses a generic span or container. This keeps the content present and gives applications a stable place to add behavior.

:product[Trail shoes]{sku=TR-42}

::: product-card
Trail shoes are available in three sizes.
:::

An application can later attach meaning to product and product-card; a plain processor still preserves their words and attributes.

Choosing between document syntax and an extension

Prefer core syntax when the meaning belongs in the document and should work in every processor. Prefer an extension when the behavior needs external data, application state, a third-party renderer, or an interactive interface.

Good extension behavior has three properties:

• the source remains understandable without the extension;
• disabling the extension does not silently discard authored words;
• unavailable interaction degrades to useful static output.

Technical reference

The extension contract defines matcher purity, transforms, rendering hooks, generated identifiers, configuration, feature tiers, and the normative behavior of the standard extensions. It is intended for extension and engine authors rather than readers choosing a feature.