From HTML5 to HTML5.1, the suggested rendering of wbr element has been changed from "content" property to "display-outside".
https://www.w3.org/TR/html5/rendering.html#phrasing-content-0
https://www.w3.org/TR/html51/rendering.html#non-replaced-elements-phrasing-content

However, display-outside for both br and wbr elements relies upon values which have been discussed on February 2015 (https://logs.csswg.org/irc.w3.org/css/2015-02-23/) and never implemented nor standardised since then.

As a matter of fact, these two elements still work because the browser knows what to do. This is relevant because there is no apparent way to change the default rendering for these elements.

I reported this issue some time ago when I suggested that wbr, as word break opportunity, could be used for a better soft hyphenation. This would allow authors to break long strings consistently and without having to insert ­ (it tends to behave erratically when it comes to deal with text content in any way). Current suggestion isn't even meant to allow that in the future.