Formerly one could use \stigma and \koppa within mathematical notation in Wikipedia articles. Perhaps only one article used them, but it does rely on them. What appears now is just blank spaces. See the section titled "The numeral system and the appearance of the untranslated table" in the article titled "Ptolemy's table of chords". Look for stigma and koppa in both of the tables. Blank spaces appear where stigma and koppa should appear.
Please always provide a link to an example - I assume this is about https://en.wikipedia.org/wiki/Ptolemy%27s_table_of_chords#The_numeral_system_and_the_appearance_of_the_untranslated_table ?
Blank spaces appear where stigma and koppa should appear.
Not here. "Math" on https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering is set to
"MathML with SVG or PNG fallback". Please provide steps to reproduce.
OK I now see that \koppa and \stigma work when I'm not logged in, but not when I am. On my preferences page under "Math" I've chosen "MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools)". I would not think that any choice of preferences should cause these letters not to be visible.
@Physikerwelt you need to check on all browsers ;-) I can reproduce this on Chrome and Web/Epiphany.
I can't find it but there was a similar report earlier.
The problem seems to be a browser bug regarding <text> elements in SVGs used as background images. I.e., https://en.wikipedia.org/w/index.php?title=Special:MathShowImage&hash=201dbb2fde10937103359420fe8d8b84&mode=5 looks fine in Chrome and Web as a standalone image but in the page the koppa won't show.