Local font fallbacks make authoring more error prone

For various purposes most people that work on their website have custom fonts installed at a system level locally. For example, most design tools like Sketch require local font installation. This side-effect of having the font installed means that fonts requested via google fonts are never downloaded on the author's browser.

The side effect of using a system font ripples through the development process, specifically:
- Because the font is not downloaded, it is easy to forget to include it in the total download size required to render the page (e.g. via the network inspector).
- Any `display=` value is effectively unused. That means most authors don't know how their site looks when `display=swap` is used or if it's even a good idea to use `display=swap`.
- Similarly to the previous point, there is no way to test the different effects of e.g. `display=swap` vs `display=fallback`.

As a potential solution, would it be possible to remove the `local` sources in Google Fonts css? Perhaps via a URL parameter? I'm not sure if it's a good idea to do this by default -- though I imagine it's not an amazing idea to change the default under everyone.