Looks interesting, but removing the underline for links and only rely on colour except on hover is a big no on the accessibility side
Who else needs underline?
imo, if you're building a new library, this level of accessibility should be baked in. Given the aim of the library to have better css defaults, expecting developers to override the link styling to add underlines or expecting the users to install browser extensions to do that, feels like a weird requirement.
Subjectively, I also don't agree that it's ugly. There are also other options like border-bottom or dashed underline or pseudo-elements with additional symbols.
(However, I'm not sure color-only link distinction is actually an issue. It's never been considered problematic in accessibility reviews of the corporate sites/apps I work on professionally.)