[webkit-dev] Request for position on Element Timing

Ryosuke Niwa rniwa at webkit.org
Tue Nov 3 00:30:59 PST 2020

On Fri, Oct 30, 2020 at 1:58 PM Nicolás Peña Moreno <npm at google.com> wrote:
> Hi, I'd like to request WebKit's position on the Element Timing API,
which lets web developers annotate images or text whose performance they
care about. They can then obtain rendering timestamps from the
PerformanceObserver. For cross-origin images the detailed information is
gated on Timing-Allow-Origin. The proposed specification is at
https://wicg.github.io/element-timing/ and is currently shipped in
Chromium. Thanks!

Apple's WebKit team reviewed this API and we have a few concerns including
but not limited to:

   - The proposed API exposes timing at which a given element is painted.
   Implemented naively, this exposes the implementation detail of what kind of
   compositing tiles are used on a given web page. Hiding this
   implementation detail and recording the exact theoretical paint timing will
   be prohibitively expensive to do on all websites.
   - The definition of the set of owned text nodes and how they compute
   intersectionRect seems inadequate. It's unclear what "border box" of
   *Text* node would mean. The spec doesn't seem to ever populate "set of
   elements with rendered text" either.
   - The use of this API seems to incur a significant runtime as well as
   memory cost.

- R. Niwa
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