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We typically build new sites using our own starter theme: https://github.com/kalamuna/kalapress-pro. KPP KalaPress has undergone an a11y audit and has been tested.
WordPress has a directory of accessibility-ready themes as well: https://wordpress.org/themes/tags/accessibility-ready/
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Consider educating those in control of the content strategy. The majority of Wordpress pages are editable blocks that should still adhere to a11y standards. Review the Site Editing Checklist [This page is a WIP] as a guide.
Forgo the trap of a11y plugins. Focus on creating a strong content strategy with a11y to begin with.
Use a starter built with a11y in mind:
KalaPress: https://github.com/kalamuna/kalapress-pro (KPP KalaPress should be used as a starter theme for the majority of Kalamuna projects).
But if you’re looking for an alternative, use Underscores: https://underscores.me/ it provides some basics like skip links out of the box!