From a web development book:
Instead of taking a moment to appreciate the calm, developers have taken advantage of the stabilizing front-end platform to pile on a whole new wave of front-end complexity
It means that until now, front ends were not very stable, which caused a lot of chaos for developers. But now, front-ends are stabilizing, so the chaos that used to be caused by the unstable front-ends is no longer a big problem for the developers.
But, instead of appreciating this moment of calm where they no longer have to worry about unstable front-ends, they've used the opportunity to start doing new things (which will probably cause their own bunch of problems, until it's not calm anymore).
In short, they solved a bunch of problems but instead of enjoying not having those problems anymore, they jumped to new things which have and will create a new bunch of problems.