Three.js camera tilt up or down and keep horizon level

camera.rotate.y pans left or right in a predictable manner.

camera.rotate.x looks up or down predictably when camera.rotate.y is at 180 degrees.

but when I change the value of camera.rotate.y to some new value, and then I change the value of camera.rotate.x, the horizon rotates.

I've looked for an algorithm to adjust for horizon rotation after camera.rotate.x is changed, but haven't found it.