old style vs proportional font

old style and proportional font both are nouns.

Word	Noun	Adjective	Verb	Adverb
old style	Yes	No	No	No
proportional font	Yes	No	No	No

As nouns, proportional font is a hypernym of old style; that is, proportional font is a word with a broader meaning than old style:

old style: a typeface (based on an 18th century design) distinguished by irregularity and slanted ascender serifs and little contrast between light and heavy strokes
proportional font: any font whose different characters have different widths

old style (noun)	proportional font (noun)
a typeface (based on an 18th century design) distinguished by irregularity and slanted ascender serifs and little contrast between light and heavy strokes	any font whose different characters have different widths