I have searched high and low for a resolution or at the very least an explanation on this one, but I can't find anything so I am resorting to a post ...
When html pages are edited directly in the Files area, on occasion, some of the fonts in the original html file are replaced with a different character or by the common little black triangle with white question mark inside. So far I have found this on 'symbol' fonts.
The original HTML file displays fine in both firefox and IE it is only when edit is clicked, that there becomes a problem and the chars are replaced. I can even cut and paste the html page into a web page resource and have it display properly with the symbol font, but when you edit directly from the Files area, the symbol is removed and replaced with unknown gobblygook.
I've done a lot of sleuthing and read about using special characters and writing back to the db, and as well that moodle only has a small subset of symbol characters in htmlarea ... and then there are utf-8 issues... I don't know if one or all of these is the problem I am having and would love some feedback if anyone can offer some...why can you edit and view the proper fonts the file in the web page resource but not edit in the files area?? (note: I also tried amending the code to allow the html editor in the files area and that just put different gobblygook in). I am pulling my hair out ... much thanks!
anyone ... ? 
I have experienced something like that once or twice. I haven't investigated but I think it is a utf-8 issue. It seems to not happen when the uploaded file is encoded utf-8. 