Add ruby annotations to show pronunciation
Ruby annotation prints a small pronunciation gloss above a run of text, most familiar as furigana over Japanese kanji. W3C attaches H62 to one criterion, 3.1.6 Pronunciation, at Level AAA, and lists it as sufficient there. It is not a Japanese-only technique, which surprises most people who meet it. Any language where the same written run can be said more than one way is in scope, and W3C's own example annotates the initialism WCAG with the pronunciation Wuh-KAG. Two markup details. The rt element carries the reading, and rp holds the fallback brackets that show the reading inline for anything that cannot render ruby at all.
How we find it in an audit
This one only comes up in content that uses it, reviewed by somebody who reads the language. Where a client publishes Japanese for a general audience we check whether ambiguous or uncommon readings carry annotations, and whether the markup includes rt for the reading and rp for the fallback.
How affected users experience it
Ambiguous readings trip up sighted readers first, especially learners, younger readers and anybody meeting an unusual compound. They guess the pronunciation and sometimes guess the meaning along with it. Ruby settles it in place, on the page, with no footnote to go and find. Where a name has to be said right, writing the reading into the sentence as well reaches everybody.
Passes vs. fails
Passes
<ruby>東京<rp>(</rp><rt>とうきょう</rt><rp>)</rp></ruby>Fails
A Japanese page that uses an uncommon reading of a personal name with no gloss, leaving readers to guess.
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 H62: Using the ruby element. 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.