Intent to implement: CSS line-break property

[Jonathan Kew]

Summary: The `line-break` property allows authors more control over
where soft line breaks are allowed, particularly for East Asian content.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1011369
https://bugzilla.mozilla.org/show_bug.cgi?id=1531715

Link to standard: https://drafts.csswg.org/css-text-3/#line-break-property

Platform coverage: Everywhere

Estimated or target release: 69

Preference behind which this will be implemented: None

Is this feature enabled by default in sandboxed iframes? Yes

DevTools bug: none

Do other browser engines implement this? Yes, both Blink (mostly
complete, except the recently-added `anywhere` value) and Webkit (less
complete, according to wpt.fyi results) have been shipping this for some
time.

web-platform-tests:
https://github.com/web-platform-tests/wpt/tree/master/css/css-text/line-break

Is this feature restricted to secure contexts? No, it's a CSS property
that has existed in other browsers (for all contexts) for a long time.