BR and WBR style refers to "display-outside" which is no longer present in CSS Display Module

[mmotorny]

br { display-outside: newline; } /* this also has bidi implications */
wbr { display-outside: break-opportunity; } /* this also has bidi implications */

See https://html.spec.whatwg.org/multipage/rendering.html#br-wbr-content.

Quote from CSS Display Module:

Removed display-inside, display-outside, and display-extras longhands, in favor of just making display multi-value.

See https://drafts.csswg.org/css-display/#changes.

[zcorpan]

I think this is basically a duplicate of #2291 (which has a PR at #2298 but needs tests to better inform what the spec should say)