Structured data validation is verification that the markup of content in the page's code is correct and complete. Errors are common, and most companies are unaware of them because outwardly they show up in no other way than the rich result failing to appear. The most frequent problems are missing required properties, a mismatch between the markup and the page's visible content, markup for content that is not on the page at all, an incorrect price or date format, and multiple contradictory markup from different plugins. Check after every major change to templates and when deploying a new plugin. Remember that correct markup does not guarantee that a rich result will be displayed – it is a condition, not an entitlement. Set up the check as a regular item in the technical audit, not as a one-off action after deploying markup, since errors arise with every template change.
See also: FAQ schema and its limits, Semantic triple, Breadcrumbs.
