Preflight is a pluggable framework for content quality assurance. Out of the box, Preflight tests general readability (including Flesch reading level; sentence length; and word length and complexity), checks links, and provides a text replacement mechanism.
Preflight is available as an Umbraco 8 content app only, and currently supports content in RTE editors, including those nested in Grid editors. Nested/Stacked content support will be added in a future release.
Refer to the repo on GitHub for documentation around building custom tests.