The reasons I originally chose this design were:

• anything can be a Modifier (such as a String, HtmlElement[IO], or Signal) without requiring implicit conversions, that are discouraged in Scala 3
• typeclasses are more the Typelevel way

But there are some trade-offs:

• they require the shapeless magic to work, so when you get it wrong the compiler errors can be quite intimidating and confusing
• typeclasses are harder to understand, and take more boilerplate to implement your own instances. see also:
  Make it easier to implement custom modifiers #203
• furthermore, they are harder to pass around. for example this is impossible

  def MyComponent(mods: Mod[HtmlElement]*) = 
    div(
      cls := "decoration",
      mods
    )

  It is slightly mitigated by changes in:
  Add modifier instance for tuple of modifiers #199
  But still not so convenient.