Responsive email
An email whose layout adapts to the screen it is opened on.
A responsive email reflows for narrow screens: columns stack, text grows, images scale, tap targets stay big enough to hit. Since most email is read on a phone, this is closer to a baseline than a feature.
Email cannot rely on the layout tools websites use. Support for modern CSS is uneven across clients, so responsive email is built from tables plus a narrow set of rules that degrade safely where they are not understood.
That is also why HTML written for the web — by a developer used to sites, or by a language model — often looks right in a browser and falls apart in an inbox.