- AirBnb[1]: "We'd always recommend that you use Google Chrome to browse the site: we've optimised things for this browser. Thanks."
- Seamless[2]: "Safari is not a good browser to use with our website. Google Chrome work better with our website or you can download the app."
- DirecTV Now[3]
- IDAT solutions[4] (some random thing I found while searching)
- Countfire[5] (ditto)
Groupon said something similar as well but walked it back. Whatsapp's web app didn't support Firefox initially but does now.
Could browse through Web Compat[6] as well, though I'm not sure if this site is more about sites that should work in FF but don't, or Firefox bugs.
[0]: https://www.theverge.com/2018/1/4/16805216/google-chrome-onl...
[1]: https://twitter.com/AirbnbHelp/status/752829250198245376
[2]: https://twitter.com/Seamless_Care/status/942798540396474368
[3]: https://www.theverge.com/2017/6/7/15758274/directv-now-googl...
[4]: https://www.idatsolutions.com/faq/
But as of now, Chrome is and has for a long time been the only browser to support:
Shadow DOM: https://caniuse.com/#feat=shadowdomv1
Custom elements: https://caniuse.com/#feat=custom-elementsv1
CSS Paint API: https://caniuse.com/#feat=css-paint-api
That's been in Chrome for, like, less than a year - hardly a "long time". And the implementation leaked history spectacularly [0]. Rushing features out the door isn't necessarily something to be proud of.
[0] https://www.spinda.net/papers/smith-2018-revisited.pdf (see section 3.1)