Adobe pioneered this technology with their Multiple Master fonts which never seemed to catch on. OpenType 1.8 reinvents the concept and CSS Fonts Level 4 adds the 'font-variation-settings' property to set to set the design axes.
I've just checked into Inkscape trunk rendering support for the CSS 'font-variation-settings' property. This requires a yet to be released version of Pango (1.41.1) and a very recent version of FreeType. The above image was created using Inkscape to render a handwritten SVG file. A GUI is yet to come and is one of the anticipated projects for our upcoming Boston and Seville hackfests (both of which I'll be attending).
Chrome and Safari already support variable fonts, Firefox nightly does too (under a flag), and support should be in the next version of Edge. Here is a the SVG file rendered above.
SVG, Variable Fonts, Inkscape, CSS Fonts