If the rectangle above is red, then the WebfontLoader did not load. You might want to load it locally. If it is green, the page can use remote (Google/Typekit) fonts. If it is orange, then this page can not use remote fonts and falls back to the self-hosted font. Turn off your Adblocker, quit your browser and reload this page. If the rectangle is still orange, then probably your ISP blocks access to the font hosting.
This plugin returns a 'promise' if the font from whatever source is loaded. Reveal.js will then continue initialisation. This demo sets opacity to 0 and then to 1 if Reveal.js is ready, to make sure that no Flash Of Unstyled Text is shown.
From the Reveal options you can: