Storefront, performance, and errors

Storefront

• Support viewports for mobile, tablet, and desktop.
• Put static storefront styling in extension-owned classes and compile it with the plugin. See Add SCSS variables. When a value is genuinely dynamic, such as a calculated progress width, constrain and escape it appropriately rather than generating unrestricted styles.
• Avoid !important unless it is unavoidable.
• Design for accessibility; the extension must not break the overall look of the store and must be responsive.

Performance

• Avoid significant storefront or server performance regressions; compare representative pages and workloads before and after activation.
• Stability under load matters for review: avoid severe performance regressions.
• Run a Google Lighthouse audit before and after activating the extension. Significant regressions in performance, accessibility, best practices, or SEO can fail review.

Errors and HTTP behavior

• No JavaScript or console errors in the storefront or checkout. Test the full Storefront with the browser developer tools.
• Valid user flows must not introduce uncaught server errors, broken links or missing assets.
• Invalid input, unauthorized access, unavailable resources and rate limits should return appropriate HTTP responses, such as 400, 401/403, 404 or 429. These controlled responses are different from an unexpected failure during a valid operation.
• Do not hide failures by returning HTTP 200 with a success message when the operation failed.
• Error messages must clearly state what went wrong or what the user should do.
• Extensions must work without uncaught 500 errors in normal operation.

Verify: test both successful requests and expected failures. Include malformed input and restricted users, and confirm rejected requests do not leave partial data changes. For authorization and state-changing requests, see Security and permissions.

Tools

• Google Lighthouse — performance and quality audits.
• Google Rich Results Test — use together with SEO and structured data when you change product or listing markup.

After uninstalling the extension, Shopping Experiences must keep working in the storefront. See Uninstallation and data cleanup for data and CMS cleanup rules.