Any time someone says this it's important to add a caveat of "and I want my site to look the same everywhere". Using @supports means you can feature detect what CSS the user's browser supports and enhance where possible. A user with an older browser might see a less pretty, simpler design with somewhat worse UX, but that's often ok if it's a tiny minority and you can give the users on new browsers a much better experience. The two versions might look a bit different but that's fine.