bug#35005: 27.0.50; Fontification unexpectedly works with anonymous faces

[Markus Triska]

The Elisp info material states in "39.12 Faces": 

      Many parts of Emacs require named faces, and do not accept
   anonymous faces. These include the functions documented in Attribute
   Functions, and the variable ‘font-lock-keywords’ (see Search-based
   Fontification). Unless otherwise stated, we will use the term “face”
   to refer only to named faces.

However, when I start Emacs with "emacs -Q", and then evaluate in
the *scratch* buffer the form:

    (progn
      (font-lock-add-keywords nil '(("hello" 0 '(:background "green")) t))
      (insert "hello"))

then I see that "hello" is inserted and highlighted in green, apparently
due to search-based fontification where an anonymous face is specified!

I am currently working on an application where this functionality (i.e.,
anonymous faces that can be specified for fontification) would be
extremely useful. Could you please consider supporting this feature,
and - if this already works as intended - officially document it?

Thank you and all the best!
Markus

In GNU Emacs 27.0.50 (build 1, x86_64-apple-darwin18.0.0, X toolkit, Xaw scroll 
bars)
 of 2018-11-15 built on mt-computer
Repository revision: b4eb908f858284a7962851fd99c94598f76afa6f
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000