Fwiw, this bug was part of why I completely misunderstood a set of diffs and posted this query:
https://www.wikidata.org/wiki/Wikidata:Project_chat/Archive/2014/04#Problematic_edits_including_Commons_category_unknown_value

To make it even more obvious, the diff could be

-CustomValue: blah
+UnknownValue

(that is unified format; obviously the layout is different for two panel diff)

In the above presentation, italics would not be necessary.

italics can be weird and not appropriate in some languages (e.g. Japanese [1]) maybe there is another option than italics? or a way for the fonts / css /whatnot to handle emphasis <em> appropriately in given language?

[1] http://www.localizingjapan.com/blog/2011/02/27/italics-in-japanese/

No need for a new design decision, there are already CSS classes in wikibase.css for exactly this case:

.wb-snakview-variation-somevaluesnak,
.wb-snakview-variation-novaluesnak

Change 195876 had a related patch set uploaded (by Thiemo Mättig (WMDE)):
Rework MessageSnakFormatterTest

https://gerrit.wikimedia.org/r/195876

Change 195877 had a related patch set uploaded (by Thiemo Mättig (WMDE)):
Make "unknown value" and "no value" snaks in diffs italic

https://gerrit.wikimedia.org/r/195877

created T92370 for the issue of italics. (it's one of those super annoyances for me, and probably for people who use wikidata in such languages)

Change 195876 merged by jenkins-bot:
Rework MessageSnakFormatterTest

https://gerrit.wikimedia.org/r/195876

Change 195877 merged by jenkins-bot:
Make "unknown value" and "no value" snaks in diffs italic

https://gerrit.wikimedia.org/r/195877