Store the user's accessible-version preference
SVR4 is attached to no success criterion, and it covers two different situations that get treated as one. The first is an ordinary preference. A viewer wants captions showing by default, and the server remembers it. Nothing about that is remedial and nothing about it implies your site is broken. The second is the conformance escape hatch, where a preference routes somebody to a conforming alternate because the primary version has problems. Both live in this technique, and only the second one usually gets described, which is why SVR4 has a worse reputation than it deserves. The requirement to build against is that the preference, or a link to where it is set, is reachable from every non-conforming page. A setting nobody can find from where they are stuck has not been offered. And the version the preference delivers has to conform as claimed, which sounds obvious and is the check most often skipped.
How we find it in an audit
Reviewers set the preference, then navigate, then come back tomorrow. Persistence is the whole test. It fails in three predictable places. It resets on a new page, it resets at login, or it survives in the session and dies when the browser closes. We also check the control that sets it is reachable from the page a stuck user is actually on, and that the control itself is operable by keyboard and announced properly. Then we look at what the preference delivers, because a preference honored perfectly is worth nothing if the version it serves is two releases behind.
How affected users experience it
Setting the same preference on every page of a site is a small cost repeated until it becomes the reason somebody leaves. Twelve pages into a task, having pressed the same switch twelve times, most people conclude the site is not for them. They are not wrong. Stored properly, the preference is a single decision that holds. That is the difference between an accommodation somebody uses and one they gave up on during their first visit.
Passes vs. fails
Passes
setcookie("view", "conforming", ["expires" => time() + 31536000, "path" => "/"]);
// Every later response reads the cookie. Every non-conforming page links to the setting.Fails
<a href="?view=accessible">Switch to the accessible version</a>
<!-- Applies to this page only. The next click puts you back where you started. -->This guide is our interpretation of W3C technique SVR4: Allowing users to provide preferences for the display of conforming alternate versions. 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.