Offer a style switcher as an alternate version
C29 is the odd one in this family, and the difference is worth knowing before you rely on it. W3C attaches it to no success criterion at all. What it serves is the alternate version clause in WCAG's conformance requirements, which lets a page meet the standard through a conforming version the reader can reach, rather than through the default presentation. A style switcher is that mechanism. A control on the page swaps the stylesheet, client side or server side, and the alternate presentation is what conforms. Three conditions carry the weight. The switch itself has to be keyboard operable and properly labeled, it has to be findable from the page it fixes, and the presentation it produces has to conform fully at the level being claimed. Fix the default presentation where you can. This route exists for the cases where you genuinely cannot.
How we find it in an audit
We operate the switcher by keyboard and check it has a name and a state. Then we audit the alternate presentation as a page in its own right, because a switcher that leads to a version with its own failures has moved the problem rather than solved it. We also check the choice survives a page load, since a setting that resets on every navigation is not a version anybody can use.
How affected users experience it
The classic failure is an unlabeled image link showing two letters, not reachable by keyboard, that forgets the choice on the next page. Somebody who needs the alternate presentation has to find and press that control on every single page, if they can find it at all. A real button, named, with the choice remembered, is the difference between a feature and a gesture.
Passes vs. fails
Passes
<button onclick="setStyle('large')" aria-pressed="false">Larger text</button>
<!-- a real button, named, and the choice is stored for the whole site -->Fails
<a onclick="setStyle('large')"><img src="aa.gif"></a>
<!-- unlabeled, not keyboard operable, forgets the choice -->This guide is our interpretation of W3C technique C29: Using a style switcher to provide a conforming alternate version. 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.