Variant fonts are an extension to the OpenType 1.8 format that lets you store an infinite number of fonts and designs in a single file. The designer can set various text options-width, slant, boldness, serif shape-and the typesetting engine draws the text with those options. Using variable fonts in web design not only gives the designer more options, it also speeds up web pages, because one variable font file weighs less than multiple files with different fonts.
People from companies like Apple, Adobe, Microsoft and Google have been involved in developing the variable font specification.
All modern browsers and graphics apps currently support variable fonts.