UrlAttrName

An attribute name whose value is an address: href, action, src.

Separate from AttrName because a Url means something only where a browser will follow it. Restricting the arm to these three is what makes Attrs.cls := Url.Mounted("/posts") a compile error rather than a class attribute that quietly moves under a mount. The constructor stays private[html] so Attrs is the one list that decides which names carry a Url, and the constructors of Attr and AttrValue.Link are package private for the same reason: either of them left public would let a call site outside io.eezo.core.html pair a name of its own with an AttrValue.Link, minting one outright or lifting one out of an href attribute it had just built, and so mint a fourth url bearing name that a mount would then rewrite.