Use standard diacritics readers can turn off
Many languages use diacritical marks to show pronunciation or to tell similar words apart. Text without them can still be readable, and adding them helps. So G163 asks for two things at once. Use the standard marks, and give the reader a way to turn them on or off. Marks nobody can switch off are their own barrier, because fluent readers find them cluttering. W3C lists this as a sufficient technique for 3.1.6 Pronunciation, at Level AAA, and we file it with the 3.1 reading and language family alongside the other pronunciation techniques. One requirement is not negotiable and is where implementations usually fail. The marks have to be real Unicode characters in the text, because marks baked into an image cannot be turned off, cannot be resized, and cannot be read aloud.
How we find it in an audit
Our reviewers look for the control first, then use it. We check that the unpointed version really is the same passage rather than a separate file that has drifted. We copy the text out and inspect it too. An image of pointed text looks identical on screen and behaves nothing like text.
How affected users experience it
A pointed study text stored as images gives a screen reader nothing to read. The passage is silent. It also gives a low-vision reader no way to enlarge the marks, which are the smallest thing on the page and the part they most need bigger. Real characters with a toggle serve the learner who needs the marks and the fluent reader who does not, from one source.
Passes vs. fails
Passes
Vowel points are standard Unicode diacritics, and a control offers pointed and unpointed versions of the same passage.
Fails
A Hebrew study text bakes vowel points into images of text, so no reader can adjust them and no screen reader can read them.
Other ways to satisfy this rule
5 guides on this site are filed under 3.1.6 Pronunciation. W3C lists this one as sufficient for that rule on its own. Implement it correctly, in a way your readers' software actually supports, and the rule is met.
This guide is our interpretation of W3C technique G163: Using standard diacritical marks that can be turned off. W3C publishes its techniques as guidance rather than as the standard, and says so on every one of them. The success criterion is what conformance is measured against, and a technique is one documented way to meet it.