A functional document (including web content) is a document that readers have to do something with. Functional documents include contracts, reports, advice, disclosures, fact sheets, policies, procedures, terms and conditions, letters. Most documents written by businesses and government agencies are functional documents.
Whenever you write a functional document you are taking on risk.
A fundamental principle of plain language is that the writer takes prime responsibility for the communication. You can’t blame the reader if they don’t get it. You may not be able to blame the reader even if they don’t read the document. The question is shifting from “Did you read the document?”, aimed at the reader, to “Is the document readable?”, aimed at the writer.
We generally consider risk by thinking about likelihood and consequence; thinking about the likelihood a reader may misunderstand your document, or not read all of it; thinking about the consequence of a reader acting on missing or confused information.
You can probably predict likely consequences. For many documents, risk will be low because the consequences of misunderstanding are not that bad.
But you can only know about likelihood by testing the document. You can only appreciate how people are likely to read and understand your document when you run it past a sample of real users and observe their reactions and thoughts.
You can prove you have taken document risk seriously by having your document certified.