It will be! We're not sure what the eventual protocol will look like, but it'll probably start with webfinger, which can then point to whatever authentication gateway you want. We only need SMTP as the base case in the chain of strategies: https://github.com/portier/portier.github.io/blob/master/Des...
At the risk of stating the obvious choosing webfinger implicitly excludes anyone who does not or can not control their website sufficiently to implement webfinger. (ie: hosted webservices similar to Tumblr et al)
Owning the domain is not the same thing as fully controlling the web service.
My use of Tumblr as an example rather than simply stating hosted-CMS has probably misconstrued my point. (I use Tumblr with my own domain and forget that it is primarily not used that way.)
Right if your domain is simply CNAMEd to a Tumblr or Blogger or similar hosted CMS you probably have no way to get webfinger in place.
Another example here is any domain that uses static hosting. Lots of domains are hosted on GitHub Pages these days as the obvious big example (but you can replace that with any other static host like Netflify or raw S3 even). The webfinger spec uses query strings to differentiate user requests which means that a static hosted site is at best limited to a single user/email account (by ignoring the query string and returning a static JSON document).
