TL;DR: The WP Accessibility plug-in contains features that will be useful to all WordPress sites, such as support for the longdesc attribute. Even if your theme was designed with accessibility in mind, there’s a place for the WP Accessibility plug-in on your site.

The most common question I get about the WP Accessibility plug-in is “When do I need this plug-in?”

Fact is, most people don’t know whether their themes are accessible. Further, it’s hard to pin down which aspects of the WP Accessibility plug-in are going to valuable for any specific theme.

And that’s a hard question to answer.

Technically speaking, every single feature implemented in the WP Accessibility plug-in could be implemented in a theme. Themes have access to all of the same WordPress APIs that plug-ins do, so everything is on the table at the technical level.

Most likely, however, no theme in the WordPress theme repository or from any other WordPress theme directory (paid or otherwise) implements all of these features. At this point, I’d be willing to place bets on that. (Anything launched after this post is published isn’t eligible for betting, though…).

I feel the need to be a bit more specific, so I’m going to run down each feature of WP Accessibility and discuss the specific likelihood you’re going to need the feature.

The features of WP Accessibility

For detailed break downs on what each feature does, read The WP Accessibility Plug-in at the Make WordPress Accessible team site.

  1. Enable skip links with WebKit support

    Skip links are moderately common in WordPress themes, but almost never implemented correctly. This is because themes that are derived from common base themes (including Twenty Twelve and Twenty Thirteen) repeat a mistake made in those themes: placing the skip link after the heading links. It’s not a crucial error, although it’s not ideal. However, skip links in WordPress themes rarely are visible on keyboard focus, so that’s something to look out for.

    All themes with the accessibility-ready tag should have conforming skip links.

  2. Add language and text direction attributes

    This is also quite common to WordPress themes. Most likely you don’t need to enable this, and if you’re operating a multi-lingual site, you probably shouldn’t.

  3. Add a :focus outline to focusable elements

    It’s not necessarily the prettiest solution, but very few themes provide keyboard focus. It is easily testable, however: tab through a chunk of your theme. Can you clearly see where you are? No? Turn it on.

  4. Toggle for high-contrast and large font stylesheets

    If your theme has this already, you should be able to tell. If your theme is already high contrast with spacious fonts, you really don’t need this — you’ll just have to judge it for yourself.

  5. Add support for longdesc attributes

    It is amazingly unlikely that any theme has support for this, and it’s certainly not part of WordPress core. There are a few settings for this: default browser support (which is essentially “none” for now), link to description, and a jQuery overlay toggle for the long description. You should probably enable this if you ever, ever, ever add an image that calls for a long description.

  6. Remove redundant title attributes

    This is a tweak of WordPress core. It’s very unlikely that most themes do this, so you can generally go ahead and turn it on. In addition, even if your theme does strip title attributes, it’s highly unlikely that having both features enabled would cause any problem.

  7. Remove the target attribute from links

    Again, very unlikely that your theme does this. It is possible that your theme or a needed plug-in actually inserts the target attribute in some cases, however; you should be attentive to whether this causes any problems. There are select cases where stripping a target attribute can cause problems. One example is informational links while filling out forms.

  8. Force a search page error on empty search

    I know for a fact that there’s at least one WordPress theme that fixes this, Neighborly, but it’s not a common feature. It’s a WordPress core fix, so it will be common to all themes, generally speaking.

  9. Remove tabindex from focusable elements

    You should probably always turn this on. tabindex can really mess things up. It’s no longer very common in themes, but a few years ago it was something I saw in a lot of themes, particularly in the comment inputs.

  10. Strip title attributes from images inserted into content

    If you started your site after the release of WordPress 3.8, you probably have no need for this. Since WordPress 3.8, title attributes are no longer a default part of the inserted HTML (HyperText Markup Language) for images. But if you used a lot of images prior to WordPress 3.8, this will filter that content and make sure those titles aren’t causing problems.

  11. Add post titles to standard read more links

    Themes that meet the accessibility-ready guidelines have this; but it’s extremely unlikely that your theme does. Turn it on!

I hope that this breakdown will help you have a sense for what use you’re likely to get out of the WordPress Accessibility plug-in.