[daveoc64]

While I am personally in favour of feature sniffing, rather than user agent sniffing, I think it's worth remembering the debacle about how the SameSite attribute on cookies was handled by the browsers a few years ago.

Several browsers shipped with an old implementation of the spec that is incompatible with the most recent, current version of the spec.

Setting the SameSite attribute to a specific value can result in the site working in newer browsers, but not in older browsers (or vice versa).

The only way to handle this is to sniff for a specific set of old browsers by user agent string, and to alter how cookies are set for those:

https://www.chromium.org/updates/same-site/incompatible-clie...

Due to the prevalence of old iOS devices that can't be updated with a more modern browser (especially iPads), the company I work for has to keep this user agent sniffing in our codebases going forward.

If the user agent string is going to be deprecated or significantly weakened, there needs to be effort among browser vendors to avoid something like this ever happening again.

[jefftk]

All the UA reduction proposals still include sending the browser major version, which is what you need to handle this kind of incompatibility.