From the spec: https://w3c.github.io/webvtt/#introduction-multiple-lines

"Line breaks in cues are honored. User agents will also insert extra line breaks if necessary to fit the cue in the cue’s width. In general, therefore, authors are encouraged to write cues all on one line except when a line break is definitely necessary."

This page uses the example from the specification:

https://people.igalia.com/fwang/webvtt-tests/subtitle-linebreaking.html

Linebreaking happens for the long (third) subtitle in Gecko, but not in Chromium. On Chromium/Linux, the subtitle goes out of the screen while on Chrome/macOS the size of the subtitle becomes smaller to remain inside the video width. I have not tried on Windows (yet).