font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
Everything else is less user friendly. This loads instantly, looks perfect on the user's screen, and is guaranteed to look exactly as it's supposed to.
By using a font like Inter UI you create the same experience for everybody, regardless of the platform.
Segoe UI, Ubuntu, Cantarell (Windows, Ubuntu, Gnome/Fedora) are within a different typographic category than San Francisco or Roboto, they're not even similar enough to each other. By using the system font stack you're basically fragmenting the user experience and look and feel.
Inter is a substitute for well crafted neo grotesque fonts loved by designers, like San Francisco (which itself is a better fitting alternative to Helvetica), it's of a similar quality.
As an aside, we will soon be able to use system-ui in place of a manually constructed font stack: https://caniuse.com/#feat=font-family-system-ui
On macOS and iOS specifically, the system font (San Francisco) has had a great deal of time and effort poured into readability — so much that rather than being a singular font, it’s a massive family of fonts or metafont with countless variants for various DPIs and use cases. For example, the “San Francisco” used for headings (h1, h2, etc) uses wider glyphs and more relaxed kerning, as is appropriate for headings and the “San Francisco” used for small text uses a variant that has its geometry tweaked to be more legible. It’s a free readability boost that all happens automatically when you use system-ui or -apple-system in your font stack. By using a custom font, you forfeit all of that.
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue", sans-serif
As I see it, this is very centric to a specific system and therefore does not offer the best user experience for everyone.
This is an amazingly well featured typeface where the author clearly has considered a number of use cases, particularly in terms of legibility at small sizes which is important for UI. This looks great! File size may be an issue for public web UIs, as someone mentioned, but there are so many more places this could be used to great effect. Will definitely consider for future projects!
The differences are very subtle (but nice).
If you decide to take this risk anyway then you should take steps to reduce the risk of future security issues. The most important thing is to use use Subresource Integrity  on your <link> tags. However, since the author doesn't provide versioned URLs (the query strings are only being used to bust the cache, not to select which file is served), this would likely cause your site to break on each release. Usually CDNs give you a URL which includes the version as part of the path.
Another issue is that SRI only checks top level resources, there's no equivalent for external fonts and images which get included from the stylesheet . It doesn't seem like the largest attack vector, but you never know what kinds of browser bugs might pop up.
Finally, Cache-Control has a max-age of 4 hours. If you don't expect these resource to change then you'd want the value to be way longer.
Also—not mentioned too prominently on the site—but it's open source as well! Awesome work. https://github.com/rsms/inter
Reasonable replacements included in most LaTeX distributions include the XCharter or libertine packages for text fonts; eulervm or newtxmath for math fonts; or mathpazo for both.
Here is quick and dirty comparison shot:
Kerning in digits looks off in their example: https://i.imgur.com/AURrPtn.png (this is an example where fixed-width is disabled), don't know how pervasive this odd kerning is, or if this is bad only in the browser. Another example of bad kerning: https://i.imgur.com/PIYJgIz.png One more example: https://i.imgur.com/bOM4MZB.png
The page mentions they have both hinted and unhinted files you can use... but they recommend only using them for Windows where users prefer that look. (You clearly do.) I'm guessing you're using the unhinted one?
So I compared the lower-case alphabet in Wordpad (in Word all fonts look shit, way to go MS), and it seems the hinting is only applied to first four letters of the alphabet.
edit: added difference image to the above album to highlight the issue
Windows 10 1809.
the st ligature is startling every time i see it, tripping me out of the flow of reading.
Apparently this is because the font has a lot of glyphs that may not exist in other optimized web fonts (relevant GitHub issue: https://github.com/rsms/inter/issues/53).
However, I didn't like the 'M' and 'W' in Ubuntu, so I think I will be suggesting that we revise the font to this neat Inter UI font, loaded with 'defer' and the system fallbacks including Ubuntu.
This font is a very neat piece of work. It satisfies the 'g' and numbers requirements and is sans-serif. I can't wait to get working with it.
Looks like apart from standard Helvetica/Apple/Microsoft fonts there’s a very small choice of fonts you can use if you have multi lingual UI.
It’s like creating a Latin font is relatively easy, but very few of them go all the way to full Unicode support.
Afrikaans Albanian Azerbaijani Belarusian Bosnian Bulgarian Catalan Croatian Czech Danish Dutch English Estonian Finnish French German Hungarian Icelandic Italian Latvian Lithuanian Macedonian Maltese Norwegian Ossetic Polish Portugese Romanian Russian Serbian Slovak Slovenian Spanisch Swedish Turkish Ukrainian Uzbek Vietnamese Zulu
I’m getting “A problem repeatedly occurred while trying to load...” after it crashes and tries to reload several times.
I think the first time I’ve ever seen that actually.
I didn't notice the navbar at the top at first too!
Yes, they are easier to design, and easier to parameterize, which is nice for you, the designer. But they are not nice for me, the reader. Using a sans font says to me that your comfort is more important than mine, and that your time is worth more than mine and everyone else's, combined.
In Firefox, I have disabled web fonts, and use Libertine everywhere. That choice is denied to me in Firefox on my phone.
However I wanted to address your last sentence. Perhaps you could try to see if the "Stylus" add-on works on Firefox mobile. I haven't tried it myself but it should.
Do you think this font would be a good choice for a text editor?
And "automatically monospacing" any font that is designed proportionally will look terrible, so stick to an entirely monospace font.
Second, they have a toggle for that, it's on the linked page.