I'd like to propose a new failure technique for SC 2.1.1 Keyboard and SC 3.2.2 On Input.

Failure description

When a keydown or keyup handler checks for the Enter key (or other keys used as IME shortcuts) without guarding against IME composition, users who rely on an IME — such as those in CJK regions — experience broken behavior. Pressing Enter to confirm an IME candidate fires keydown, which accidentally triggers the application's action (e.g., form submission) before the user has finished composing their input.

• SC 2.1.1: The keyboard interface becomes unreliable for IME users, as the IME's own keyboard shortcuts collide with the application's shortcuts.
• SC 3.2.2: An unexpected change of context (e.g., form submission) is triggered by input that the user intended solely for the IME.

Failure example

// ❌ Failure — triggers during IME composition
inputElement.addEventListener("keydown", (event) => {
  if (event.key === "Enter") submit();
});

Sufficient techniques to avoid this failure

// ✅ Option 1 — guard with isComposing (+ keyCode 229 for Safari)
inputElement.addEventListener("keydown", (event) => {
  if (event.isComposing || event.keyCode === 229) return;
  if (event.key === "Enter") submit();
});

// ✅ Option 2 — use the form submit event instead
formElement.addEventListener("submit", (event) => {
  event.preventDefault();
  submit();
});

The keyCode === 229 check is necessary to cover a known Safari bug where compositionend fires before keydown, causing isComposing to be false on the final confirmation keypress. MDN also recommends this workaround.
note: This bug has been fixed in a recent Safari version, but users on older versions remain affected. Therefore, adding || event.keyCode === 229 as a fallback is still recommended to ensure correct behavior across the full range of Safari versions in use.

References

• New lint rule: require IME-safe keyboard event handling · biomejs/biome · Discussion #10041
• If You Handle Enter, Check event.isComposing - DEV Community
• Stop implementing “Enter to submit” by listening directly to raw keydown events.
• Your Form Submits Mid-Japanese Input | A static site to link people to when their web form submits upon the user confirming their IME input.
• #45371 (Add tag: Use event.isComposing to support input method editor (IME) converters) – WordPress Trac
• https://x.com/p_craft/status/2044418886175793154?s=20