[NavigationTiming2] prerenderSwitch Attribute

One of the goals of Navigation Timing is to provide site owners real user data on the latency of page loads. The prerender feature, which allows the user agent to open a page in a hidden tab if it is highly likely that the user will visit that page, will decrease the perceived page load latency from a user's point of view. It's reasonable that this information be passed along to the site owner.

I brought this topic up in last week's conference call. Accordingly, I have updated the Navigation Timing L2 specification [1] with a new prerenderSwitch attribute. This attribute gives the time immediately after the user agent changes the visibilityChange attribute from "prerender" to "visible". If the navigation was not initiated by a prerender hint, this attribute returns 0. Likewise, "prerender" is defined as a new value for the type attribute that defines the types of navigation. I have also defined a NavigationTypes enum for all of the navigation type strings.

While site owners can use the duration attribute to understand the total page load latency (e.g., difference between loadEventEnd and fetchStart), site owners can also determine the perceived page load by taking the difference between loadEventEnd and the new attribute 'prerenderSwitch'.

I expect that as we define the Prerender specification, Navigation Timing L2 will link to that definition of prerender.

Please do provide feedback on this update.

Thanks,
Jatinder

[1] https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/NavigationTiming2/Overview.html