The problem starts here:
When text requires reading ability more advanced than the lower secondary education level after removal of proper names and titles, supplemental content, or a version that does not require reading ability more advanced than the lower secondary education level, is available. (Level AAA)
Results? To understand this sentence you would need to have 22.8 years of education, according to the Flesch-Kincaid Grade Level test of readability. Way beyond lower secondary education. And hey, I’ve had only 16 years of formal education, so it’s officially too hard for me.
The Flesch-Kincaid Reading Ease score of the sentence is 4.3. That means, roughly, that 95.7% of English-speaking adults would find the sentence very difficult to read and understand.
No wonder we’re stuck.
Content should be written as clearly and simply as possible.
That statement introduces The Intent of this Success Criterion. It’s a given, something everyone agrees with. Accessibility depends on plain language! But that’s not perceived as testable, hence the makeshift criterion.
A plain language version of SC 3.1.5
The vast majority of web content must score between 7 and 9 on the Flesch-Kincaid Grade Level (FKGL) readability test.
Any web content that scores higher than 9 on the FKGL Readability test has an alternative version, or extra content explaining the original. The new content must score between 7 and 9 on the FKGL readability test.
Alternative or supplementary easy-peasy web content is only necessary for abstruse texts that may not be rewritten, e.g. for copyright or legal reasons. Anything else: rewrite it in plain language so that it scores between 7 and 9 on the FKGL readability test.
Never mind the criterion: look at the level!
Readable content is presented as a very high level goal, more difficult than levels A and AA. Some governments do not require web sites and intranets to reach level AAA — yet obviously, if your content is unclear and unreadable, all your accessibility efforts go to waste.
History of the reading level criterion
The WCAG Working Group accepted that using the clearest and simplest language appropriate is highly desirable, but they could not find a way to test objectively whether this has been achieved.
It’s true that “it will not be possible to write a text that will suit the abilities of all people with literacy and comprehension problems.” Consequently, clear and simple language virtually went into the too-hard basket.
Disclaimer: Everything I say in this article could be off the mark, because (remember) I am officially too uneducated to understand the criterion.