To make written web content accessible, it should ideally be as clear and simple as possible. One word, plain as in plain language, means both clear and simple. But what we often forget is that these are two entirely different qualities.
WCAG 2.0 success criterion 3.1.5 settles for simplicity, the quality that is objectively testable. If a text requires reading ability no more advanced than the lower secondary education level, it is deemed to pass the criterion.
Readability tests deliver a verdict on how simple the language in a text is. They are extremely useful (if only people used them) — but they are only half the story.
A sentence can be very simple, and yet utter nonsense, like this:
Be simple words can like and utter this yet nonsense a sentence.
The above sentence is theoretically readable. Its Flesch-Kincaid Reading Ease score is 67.8. With a Flesch-Kincaid grade level of 6.8, someone with a lower secondary level of education could, in theory, read and understand it.
Of course most people don't write nonsense, so a good readability score is generally a sign that a text is also clear. But let's not forget that a readability test does not guarantee clarity.
No wonder the WCAG 2.0 committee found this impossible.
All the plain language audit templates I've seen all assess clarity by testing whether a document meets certain commonly accepted guidelines for plain language. Here's a typical plain language audit tool from North West Territories: see pages 7 and 8.
This and all similar documents are doubtless useful guidelines for content writers. They are guidelines, and they do not produce an objective test of clarity.
Only the first item in this typical plain language audit attempts to measure whether information is accurate and true and clear.
The purpose of the document is clear. Readers know right away why they should read the document.
Only usability testing with readers in the target audience can test whether the purpose (and the content) is clear. This is not something that an auditor can just decide on and tick.