Should Unearthed Arcana articles be considered solid or work in progress?

@mikemearls should UA articles be considered solid, or WIP. If the latter, in what form will these be made into official rules releases?

— Monument Games, Inc (@MonumentGames) January 8, 2016

They are works in progress – they'll appear in official form if people seem to like them and after playtest revision https://t.co/t4dvh9UVno

— Mike Mearls (@mikemearls) January 11, 2016

@DropTheDie @MonumentGamesif I get “Shadow Bloodline Sorcerer” tattooed on me, will you promise to release it officially someday? I promise that you won't need to go to that sort of extreme!

— Mike Mearls (@mikemearls) January 11, 2016