I know I’m not the only one who gets super annoyed by content jumping around while I’m reading it - and I’d never intentionally create that experience for my users. But sometimes you just don’t know how your code behaves “in the wild”, and you can’t exactly ride-along with every user. The Layout Instability API aims to address this issue:

How a site functions in development is often quite different from how users experience it in production: personalized or third-party content often doesn’t behave the same in development as it does in production, test images are often already in the developer’s browser cache, and API calls that run locally are often so fast that the delay isn’t noticeable.

The first step toward properly solving this problem is to give developers the tools to measure it and understand how often it’s occurring for real users. The Layout Instability API, currently being incubated in the WICG, aims to address this.