Available since v0.1.0

Fires on the native error event of a media element — broken image, failed video load, iframe navigation error. Use for visible-failure assertions on resources you depend on.

Syntax

<img src="/hero.webp"
  fs-assert="home/hero-fallback-shown"
  fs-trigger="error"
  fs-assert-visible=".hero-fallback">

When to use it

• Resources where a broken fetch would be a visible failure (hero, avatar, product image)
• Verifying fallback UI kicks in when a media resource fails
• Dead-link detection for iframes or embedded players

Example

<img src="/user-avatar.png"
  fs-assert="profile/avatar-broken-shows-initials"
  fs-trigger="error"
  fs-assert-visible=".avatar-initials">

Pairs well with

• fs-assert-visible — fallback element becomes visible on error
• fs-assert-added — fallback element is added when the media 404s
• Conditional assertions — pair with load for success/failure branches

Gotchas

• Silent CORS failures. Some network errors fire error, others (especially CORS) fire with no useful detail. The trigger fires in both cases, but the observable failure mode differs.
• Synthetic monitoring noise. Bot traffic and link checkers may trigger this constantly. Scope assertion keys to user-facing paths.

See also

• load — the success counterpart
• Triggers index