By special characters do you mean non-ASCII characters like letters with accents and cedillas (e.g. ê, ç)? Then it is likely a character encoding issue. Linux usually assumes UTF-8 encoding whereas DOS uses code pages which specify a different characters for the 128-255 range, usually 437 for US English and 850 for Western Europe.

It's not a font issue since, as you say, it looks fine in DOS. Either set the text editor to use UTF-8 (probably impossible as period correct editors don't support that) or convert the file to UTF-8 in Linux. Some Linux text editors like KWrite can interpret the open text file as different character encodings (second option from the right in the status bar at the bottom) and then save it as a different encoding (File..Save As with Encoding).

Hello, how are you? Sorry for the delay.
I came to the conclusion that the problem is really the font, because I opened an .html file in Fed (text editor) within Dosbox and the special characters were bugged, and I also came to this conclusion due to the fact that, in the Dosbox configuration file, it changes the keyboard layout and not the font.

So I really will have to change the font, and even after searching a lot on the internet I couldn't find it, so, does anyone please know how to do this, that is, change the font in Dosbox? Thanks.

Which character encoding does the HTML file use? I tried keyb br in DOSBox and it loads code page 858, essentially 850 with the Euro (€) symbol. You can change the code page with keyb br 850 or a different code page. FED does not support UTF-8, you have to convert the encoding of the HTML file to a code page that DOS supports (and then make sure the metadata specifies the correct code page so that it displays correctly in a web browser), if you really want to edit it in DOS.

Even if a DOS text editor supports UTF-8, it'll have to use graphical mode because text mode only supports 8-bit characters, which isn't sufficient to display the wide variety of Unicode characters.

Back in the late 90s/early 2000s it wasn't uncommon to use HTML entities.
Especially since popular OSes like Windows 98 didn't support Unicode yet.

https://en.wikipedia.org/wiki/HTML_entity
https://www.freeformatter.com/html-entities.html