Structured data is machine-readable markup for visible page facts; valid markup can aid understanding but never guarantees a rich result.
Structured data turns facts already present to readers—such as product names, article authors or dates—into a machine-readable graph. JSON-LD is common, but fidelity matters more: the graph must match visible content.
Practical implementation
Choose the narrowest type that represents the page, identify stable visible facts and generate the graph from the same source of truth where possible. Validate syntax and required properties, then inspect relationships, IDs and values in the actual response.
What to verify
Compare every key marked-up value with the rendered page and test the canonical URL after deployment. Watch for stale facts, missing images, duplicate entities and markup injected after content changes.
Common mistakes
Do not add types that describe features the page does not offer or promise rich results because JSON parses.