RDFa

RDFa uses a minimal set of HTML attributes to express RDF triples. The vocab attribute declares a default vocabulary (e.g., http://schema.org/), typeof specifies the class of an entity (e.g., Person), and property defines a relationship or attribute (e.g., name). This allows any visible content to become a semantic node without duplicating data.

• Example: <div vocab="http://schema.org/" typeof="Person"><span property="name">Ada Lovelace</span></div>
• Key Benefit: Metadata lives inline with content, reducing the risk of data drift between visible text and hidden markup.

RDFa supports the @prefix attribute to define shorthand mappings for long vocabulary URIs, making markup cleaner and more readable. Instead of repeating full URLs, authors declare prefixes once and use prefix:term syntax throughout the document.

• Example: prefix="schema: http://schema.org/" enables <span property="schema:name">
• Use Case: Essential when combining multiple vocabularies like Dublin Core, FOAF, and Schema.org in a single document.
• Standard Prefixes: RDFa Core defines initial contexts for common vocabularies (rdf, rdfs, xsd), reducing the need for explicit declarations.

RDFa provides explicit control over the subject of a triple. The @about attribute sets the subject to a specific URI, while @resource creates a reference to another entity without making it the current subject. This enables complex entity linking within a single HTML element.

• @about: <div about="#ada"><span property="name">Ada</span></div> — the subject is #ada.
• @resource: <a property="knows" resource="#babbage">Charles Babbage</a> — creates a relationship to #babbage without changing the current subject.
• Inheritance: If no @about is specified, the subject defaults to the parent element's subject or the document base URI.

RDFa preserves the full expressivity of RDF by supporting explicit datatype and language annotations. The @datatype attribute specifies the literal's type (e.g., xsd:date, xsd:integer), while @content overrides visible text with a machine-friendly value.

• Date Example: <time property="birthDate" datatype="xsd:date" content="1815-12-10">December 10, 1815</time>
• Language Tagging: Standard HTML lang attribute propagates to RDFa triples: <span property="title" lang="fr">Ingénieur</span>
• Content Override: Use @content when the displayed text differs from the canonical machine value, such as formatted currencies or abbreviated dates.

RDFa naturally expresses complex, multi-entity relationships through HTML nesting. Each nested element with a new @typeof creates a new blank node or named entity, automatically connected to its parent via the property specified on the child element.

• Address Example: <div typeof="Person"> <span property="name">Tim Berners-Lee</span> <div property="address" typeof="PostalAddress"> <span property="addressLocality">Boston</span> </div> </div>
• Result: Creates a Person entity with a PostalAddress node connected via the address property, all without explicit node IDs.

RDFa embeds metadata directly within existing HTML elements, while JSON-LD isolates structured data in a separate <script> block. RDFa's inline approach ensures that the visible content and its semantic meaning are inherently synchronized, reducing maintenance overhead for content-heavy sites.

• RDFa Advantage: No duplication — the visible text is the data value.
• JSON-LD Advantage: Easier to inject dynamically via JavaScript and preferred by Google for most rich results.
• Hybrid Strategy: Use JSON-LD for primary entity definitions and RDFa for granular inline annotations where content-to-metadata alignment is critical.