Keep a conforming fallback for a scripted page
SCR38 is unusual and you should know how before you rely on it. W3C references it from no Understanding document, so it is attached to no success criterion at all. What it documents is the machinery behind a conforming alternate version, which is part of how a conformance claim is made rather than a rule you pass. The method is progressive enhancement taken literally. Build on an HTML foundation that works, keep that pre-enhanced version rather than throwing it away at build time, layer the rich experience on top, and give users a way back down to the baseline. The published test asks two things. That the enhanced page carries a link to the conforming version, and that the conforming version genuinely conforms at the level you are claiming. It asks nothing about whether the alternate is current, which is where these arrangements almost always come apart. Four other techniques do the same job by other means, and none of them is attached to a criterion either.
How we find it in an audit
Reviewers go looking for the alternate the way a stranger would, from the page that needs it, and then check whether it is worth arriving at. Two questions carry the weight. Does it hold the same information and offer the same functions, and was it built from the same content as the page it stands in for, rather than regenerated two releases ago and left there. We check the link is on the non-conforming page rather than buried in a footer somewhere else. And we say plainly what we think, which is that one accessible site beats two sites where one of them is always behind.
How affected users experience it
An alternate version is a promise that the same task can be finished somewhere else. When it holds, it holds. When it drifts, the person who needed it is the last to know, because nothing on the accessible page says the prices are three months old or the new booking step is missing. They complete a task that no longer exists in the form they completed it. The failure looks like success right up to the point where somebody phones to say their order never arrived.
Passes vs. fails
Passes
<table id="figures"><!-- Every number the chart plots, in real markup --></table>
<script src="dashboard.js"></script>
<!-- Script draws the chart over the table. The data was always there to read. -->Fails
<div id="app"></div>
<script src="dashboard.js"></script>
<!-- The entire dashboard is drawn in canvas. With script unavailable the page is empty. -->This guide is our interpretation of W3C technique SCR38: Creating a conforming alternate version for a web page designed with progressive enhancement. 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.