Theme App Extension
A Theme App Extension is a Shopify integration model that lets apps inject Liquid blocks (`app blocks`) and storefront snippets into Online Store 2.0 themes without modifying the theme source.
Detailed explanation
Theme App Extensions replaced the deprecated ScriptTag API for adding storefront features. Each extension can register `app embed` blocks (loaded site-wide on every page) and `app section` blocks (drag-droppable into specific sections by the merchant). Extensions have hard limits — 25 settings per block, 100KB total Liquid, no checkout access — to keep theme builds fast and predictable.
For accessibility apps, theme app extensions are the right tool for runtime helpers (focus traps, live regions, skip links injected on every page) — but they are not a substitute for fixing the theme source. Source fixes are durable and survive theme switching; extension fixes apply only while the app is installed.
How this applies to Shopify stores
AccessComply ships two app embeds: `a11y-fixes` (runtime focus management, skip links, ARIA live regions) and `feedback-widget` (in-storefront accessibility feedback collection). The bulk of fixes still go to theme source via `themeFilesUpsert`; the embed handles only what genuinely needs runtime behavior.
Primary source: shopify.dev