Here is the Wiktionary definition of the word "locus": https://en.wiktionary.org/wiki/locus#Noun

When I open that page in EWW, I see five definition entries. But the second entry is blank. I think: what is that missing definition, and why is it missing?

When I open it in Firefox, I see that there are only four definitions, the same ones as displayed in EWW. So EWW is not failing to display anything; instead, it is inserting something extra. That extra something comes from this piece of HTML:

  <li class="mw-empty-elt"></li>

I don't know why that's in there or how Firefox knows not to display it. It would be cool if EWW also knew not to display it.

Bug report or feature request? You be the judge.