Chrome Version: 64.0.3254.0 (Developer Build) unknown 32-bit
OS: Android 7.0.0; Pixel C Build/NRD90R

What steps will reprodkuce the problem?
(1) Go to https://w3c.github.io/uievents/tools/key-event-viewer.html on an Android device with Gboard as the active IME.
(2) Tap "Show Options" and turn on preventDefault for compositionstart, compositionupdate, and compositionend.
(3) Try typing in the text box using Gboard.

What is the expected result?

Typing should be impossible since Gboard should be reset every time it tries to open or update a composition.

What happens instead?

Typing works as normal.

We should either declare these events as non-cancelable, or properly support preventDefault().