In the context of general "-system-✻" font & style identifiers (not merely -apple-system-✻ specific)... 1. Knowing extremes and layout boundary conditions: You

In the context of general "-system-✻" font & style identifiers (not merely -apple-system-✻ specific)... 1. Knowing extremes and layout boundary conditions: You can reasonably test extremes of wide and condensed font metrics; it's not that hard. 2. Rigid designs: If you don't want to use the ambiguous system font, you don't have to; it doesn't change that. If you have a design that requires certain rigidity and inflexibility, you can have your explicitly imported web fonts and be happy. 3. Flexible designs: Be flexible and fluid. Be like water. Heck, there are slight unknowns with using any non-explicitly defined font. Using CSS generic-family identifiers (serif, sans-serif, cursive, fantasy, and monospace) have variation and are not guarantees, but you can still use them without much concern for layouts that are designed to be appropriately accommodating. 4. Native look and feel: If you want to achieve something closer to the native system look and feel, being able to use the system font from CSS helps you out immensely. For those that (1) design their layout to flow relatively nicely without being overly fixed or constrained and (2) want to have an appearance matching the system font, then it's great! 5. Testing: From my experience, this doesn't pose any significant testing issues. Besides testing artificial extremes of wide and condensed font metrics, practical testing across multiple environments isn't anything new. It's something that already gets done by any skilled front-end designer. Many shops will test across many browsers under may platforms anyway. Sites like http://browsershots.org http://browsershots.org, https://www.browserstack.com/screenshots https://www.browserstack.com/screenshots, http://www.crossbrowsertool.com http://www.crossbrowsertool.com, http://www.browsershots.atm http://www.browsershots.atm, https://www.browserling.com https://www.browserling.com, http://dev.modern.ie/tools/screenshots/ http://dev.modern.ie/tools/screenshots/ and many others are out there for people who don't have or don't want to deal with the setup in-house to do testing. (You might have to supplement it with mobile devices in-house, sure.) You can easily cover 95% of the actually used environments (browsers/platforms) very explicitly, and for the environments you don't explicitly test for, at that point of successful testing, it's very likely there won't be an issue, and if there is an issue, it should be minor and not significantly matter. Be pragmatic.