In my en Wikipedia Watchlist, when I hover over a "D" on the left of a history item, a small pop-up box appears with an explanation of the symbol. However the explanatory text shown in the box reads:
Edit made at Wiki<span style="text_decoration: underline;">d</span>ata
Obviously the intent is to display the word "Wikidata" with an underlined "d", but the box isn't rendering the underline and the text actually displayed would be very confusing to a non-techie. Hover-over boxes for other symbols (x and m) do not have a similar problem.
The simplest fix would be to get rid of the markup. I think our readers can figure out that the D stands for data without the underline hint.