Mediawiki 1.26wmf23
Using a span element in the heading of an article like this:
=== line <span style="color:green">break</span> ===
causes a line break in mobile view. This can be reproduced on
de.m.wiktionary.org and en.m.wiktionary.org

The reason is a rule in https://de.m.wiktionary.org/w/load.php?debug=false&lang=de&modules=mediawiki.ui.button%2Cicon|skins.minerva.base.reset%2Cstyles|skins.minerva.content.styles|skins.minerva.tablet.styles&only=styles&skin=minerva&target=mobile

the rule is

.content .in-block span
  display: table-cell;

I dont know what this rule is used for, fact is, if this rule is dropped, the line break does not appear any longer.