I believe it used to be in HTML5, then CSSOM, and it was removed in 2013.

Punting to CSS since it's a style-related API.

CL to add use counter:
https://chromium-review.googlesource.com/c/chromium/src/+/745322

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/708cfbd0e177072433ccf56307f0982101502af5

commit 708cfbd0e177072433ccf56307f0982101502af5
Author: Eric Willigers <ericwilligers@chromium.org>
Date: Tue Oct 31 08:02:23 2017

Document: Use counters for stylesheetSet attributes

We add use counters for the following attributes:
attribute DOMString? selectedStylesheetSet;
readonly attribute DOMString? preferredStylesheetSet;

BUG= 690600 , 690609 

Change-Id: I0edf7fc6ec5e3e9295d36f18ad5853ffabfa6c1e
Reviewed-on: https://chromium-review.googlesource.com/745322
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Bugs Nash <bugsnash@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512775}
[add] https://crrev.com/708cfbd0e177072433ccf56307f0982101502af5/third_party/WebKit/LayoutTests/fast/dom/document-preferredStylesheetSet-use-counter.html
[add] https://crrev.com/708cfbd0e177072433ccf56307f0982101502af5/third_party/WebKit/LayoutTests/fast/dom/document-selectedStylesheetSet-use-counter-001.html
[add] https://crrev.com/708cfbd0e177072433ccf56307f0982101502af5/third_party/WebKit/LayoutTests/fast/dom/document-selectedStylesheetSet-use-counter-002.html
[modify] https://crrev.com/708cfbd0e177072433ccf56307f0982101502af5/third_party/WebKit/Source/core/dom/Document.cpp
[modify] https://crrev.com/708cfbd0e177072433ccf56307f0982101502af5/third_party/WebKit/Source/core/dom/Document.idl
[modify] https://crrev.com/708cfbd0e177072433ccf56307f0982101502af5/third_party/WebKit/public/platform/web_feature.mojom
[modify] https://crrev.com/708cfbd0e177072433ccf56307f0982101502af5/tools/metrics/histograms/enums.xml

Marking as BlockedBug as we need to get use counter data.

I'm guessing we'll get enough data around April 2018.

The NextAction date has arrived: 2018-04-01

Merging this into the same bug as the two methods should be discussed at the same time.