Email clients can be broadly categorized into web-based (like Gmail, Yahoo Mail), desktop-based (like Outlook, Apple Mail), and mobile-based (like iOS Mail, Android Mail). Each of these clients has its own way of handling CSS, and some might strip out certain styles or not support them at all.
Web-based Clients: These often have better CSS support but can strip out external stylesheets. Desktop Clients: Outlook, in particular, is known for poor CSS support and can be tricky to work with. Mobile Clients: These generally have good CSS support but vary significantly between iOS and Android.