Mark Otto and Jacob Thornton (aka @mdo and @fat), who jointly cooked up front-end Web app development toolkit Bootstrap whilst working at Twitter, have decided to leave the social networking company after 2.5 years.

In a blog post, Otto and Thornton promise that they will both continue to work on the Bootstrap project, which was initially announced back in August 2011.

Bootstrap has “grown beyond” them, Otto writes, which prompted them to start moving it into its own open-source organization (for better or worse).

From the Hacker News comment thread:

The timing has nothing to do with a disagreement about Bootstrap (seriously, none what so ever), and more to do with us both wanting a change in our own lives for what we do day-to-day. Twitter, the company and product, are both amazing and Jacob and I have worked there for 2.5 years.

We’re stoked for our next things and we both want to keep working on Bootstrap no matter what. We have an obligation to the community and know it could go much further. (Oh, and yes, I screwed up the date on the post. My bad, yo.)

Bootstrap was created by me at Twitter as a means to make better looking internal tools. … It started off as a simple HTML/CSS thing, then Jacob built plugins on top of it, and we open sourced it together.

We made it at Twitter, so when we wanted to open source it, we went to Twitter to ensure it was good to go. Thus, it was named Twitter Bootstrap (originally, “Bootstrap, from Twitter” actually).

Now, it’s back to just “Bootstrap”.

Just a few weeks ago, Otto and Thornton announced the release of version 2.1, bringing simplified documentation and a ton of fixes.

For now, Bootstrap will remain a Twitter project on GitHub. Coincidentally, GitHub is also where Otto will be moving to, as he notes in a comment on the Hacker News thread about his and Thornton’s departure from Twitter.

Thornton will go his separate way and join Obvious, the mini startup incubator started by Twitter co-founders Biz Stone and Evan Williams along with early employee Jason Goldman.

We’ll keep track of what happens with Twitter Bootstrap in the future.

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Image credit: Oli Scarff / Getty Images