Re: [w3ctag/design-reviews] Layout Instability Metric (#393)

Thanks for the feedback!

We see this metric as part of a set of metrics, alongside First Input Delay (FID) and First Contentful Paint (FCP), that provide insight into user experience.  Developers should not optimize for any single metric to the exclusion of others, but should optimize for all of them together.

We have found that web fonts are a common source of layout instability.  I think platform improvements are needed to support web fonts with stable layout.  There has been some discussion of this, e.g. on crbug.com/898083.  The score can also vary with things like ad selection.

The layout shift score is generated for every rendering update that contains layout shifts.  These updates are throttled to the speed of animation frames (vsync).  There could still be a performance impact if the developer does something expensive in the PerformanceObserver callback.

A website has many ways to deduce information about the user's computing capabilities for purposes such as fingerprinting.  The layout instability metric is one such way, but we don't think it exposes information that couldn't be just as easily obtained through other APIs that measure the speed or query the state of layout.

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