I don't think lines of code or number of files are particularly revealing metrics here. We could add a large external PHP library (and have — two are 15,000 and 20,000 lines, respectively) and write a new feature powered by 5,000 of lines JavaScript. That's 7x more PHP, but the effort was clearly spent on the JS.

And it's not just about external libraries, either. It's about where our focus and effort is going. Pretty much every single user feature of the last seven or eight versions wasn't just JavaScript-heavy, but JavaScript-centric. Think the customizer in 3.4, media in 3.5, revisions in 3.6, the new themes browser in 3.8 (for installed themes) and 3.9 (the installer), tons of improved editing/media tools in 3.9. The list goes on, and will continue to grow. While PHP certainly backs a significant amount of this, it's just the piping and the foundation. We've gotten really good at pouring concrete, so laying a foundation is easy. The focus and energy needs to be on the house that gets built.

If you count lines of code, you'll probably never hit 90 percent. But if you count ongoing development efforts, I could certainly see our frontend development eclipsing our backend development. And in many ways, it already has.

The headline here is, possibly accidentally, a perfect way to frame it: in the next few years, 90% of WordPress development could be JavaScript-based. With that metric, we're easily halfway there.

(I posted this on the original blog post but it looks like the comment is still awaiting moderation.)