Even Glk, which has styles meant for meaning and use rather than formatting, nevertheless has a monospace style, since some things need to be displayed using a

Even Glk, which has styles meant for meaning and use rather than formatting, nevertheless has a monospace style, since some things need to be displayed using a monospace font to be displayed correctly. However, you could just as well display all text using a monospace font, if you are using only one font, so you might not be able to "signal specific things" by the use of a monospace font. For the case of headings, this can be done without specifying the font yourself; if you specify it is a heading, then the rendering software can decide how to display it. (HTML has <H1> and <H2> and so on for headings, and Glk has style_Header and style_Subheader.)