Non-negotiable
Specific browser required for client
Default
Latest Chrome (Mac/Win), Latest FF (Mac/Win), Latest Edge (Win), Latest Safari (Mac), Latest iOS safari (iPhone SE, 350px screen-size, and above), Latest Android Chrome. Non-bleeding-edge versions of same browsers should be supported so long as they support all features used heavily by the site.
Advanced
IE11, by specific request of the client
Extreme
Other specialty browsers