Use Counters

Use counters are used to report statistics on how much a given web platform feature is used across the Web. Supported features include:

  • WebIDL methods and attributes (getters and setters are reported separately) for pages, documents, and workers,

  • CSS properties (including properties that aren’t in the web platform, but we’re interested in),

  • Deprecated DOM operations,

  • Other things like SVG filters and APIs specifically unsupported in Private Browsing Mode, via custom use counters.

Adding a new Use Counter

How you add a new use counter is different depending on what kind of web platform feature you’re instrumenting. The one constant is that you must run ./mach gen-use-counter-metrics after adding or removing a use counter.

(Why this is a manual step and not part of the build is explained in the implementation bug 1852098.)

WebIDL Methods and Attributes

Use counters for WebIDL Methods and Attributes are added manually by editing UseCounters.conf or, for workers, UseCountersWorker.conf, and by annotating the WebIDL Method or Attribute with the [UseCounter] extended attribute.

(Why you must write this in two places is because generating things from bindings codegen and ensuring all the dependencies were correct proved to be rather difficult)

Then run ./mach gen-use-counter-metrics and build as normal.

CSS Properties

Use counters for CSS properties are automatically generated for every property Gecko supports.

To add a use counter for a CSS property that isn’t supported by Gecko, add it to

Then run ./mach gen-use-counter-metrics and build as normal.

Deprecated DOM operations

Use counters for deprecated DOM operations are declared in nsDeprecatedOperationList.h. To add a use counter for a deprecated DOM operation, you’ll add an invocation of the DEPRECATED_OPERATION(DeprecationReference) macro. The provided parameter must have the same value of the deprecation note added to the IDL file.

See bug 1860635 for a sample deprecated operation.

Then run ./mach gen-use-counter-metrics and build as normal.

Custom use counters

Custom use counters are for counting per-page, per-document, or per-worker uses of web platform features that can’t be handled directly through WebIDL annotation.

For example, the use of specific SVG filters isn’t a WebIDL method or attribute, but was still an aspect of the web platform of interest.

To add a custom use counter, define it in UseCounters.conf or, for workers, UseCountersWorker.conf by following the instructions in the file. Broadly, you’ll be writing a line like custom feBlend uses the feBlend SVG filter.

Then, by running the build as normal, an enum in enum class UseCounter will be generated for your use counter, which you should pass to Document::SetUseCounter() when it’s used. Document::SetUseCounter() is very cheap, so do not be afraid to call it every time the feature is used.

Take care to craft the description appropriately. It will be appended to “Whether a document “ or “Whether a shared worker “, so write only the ending.

The processor scripts

The definition files are processed during the build to generate C++ headers included by web platform components (e.g. DOM) that own the features to be tracked.

The definition files are also processed during ./mach gen-use-counter-metrics to generate use_counter_metrics.yaml which generates the necessary Glean metrics for recording and reporting use counter data.

This script is called by the build system to generate:

  • the UseCounterList.h header for the WebIDL, out of the definition files.

  • the UseCounterWorkerList.h header for the WebIDL, out of the definition files.

Contains methods for parsing and transforming use counter definition files, as well as the mechanism that outputs the Glean use counter metrics definitions.

Data Review

The concept of a Use Counter data collection (being a web platform feature which has the number of pages, documents, workers (of various types), or other broad category of web platform API surfaces that use it recorded and reported by a data collection mechanism (like Glean)) was approved for opt-out collection in all products using Gecko and Glean in bug 1852098.

As a result, if you are adding new use counter data collections for WebIDL methods or attributes, deprecated operations, or CSS properties: you almost certainly don’t need a data collection review.

If you are adding a custom use counter, you might need a data collection review. The criteria for whether you do or not is whether the custom use counter you’re adding can fall under the over-arching data collection review request. For example: a custom use counter for an SVG filter? Clearly a web platform feature being counted. A custom use counter that solely increments when you visit a social media website? Doesn’t seem like it’d be covered, no.

If unsure, please ask on the #data-stewards channel on Matrix.

The Data

Use Counters are, as of Firefox 121, collected using Glean as counter metrics on the “use-counters” ping. They are in a variety of metrics categories of use.counter.X which you can browse on the Glean Dictionary. The dictionary also contains information about how to view the data.

Interpreting the data

A use counter on its own is minimally useful, as it is solely a count of how many (pages, documents, workers of a specific type, other web platform API surfaces) a given part of the web platform was used on.

Knowing a feature was encountered 0 times across all of Firefox would be useful to know. (If you wanted to remove something). Knowing a feature was encountered more than 0 times would be useful. (If you wanted to argue against removing something).

But any other number of, say, pages using a web platform feature is only useful in context with how many total pages were viewed.

Thus, each use counter has in its description a name of another counter – a denominator – to convert the use counter into a usage rate.

Using pages as an example, knowing the CSS property overflow is used on 1504 pages is… nice. I guess. But if you sum up use.counters.top_level_content_documents_destroyed to find that there were only 1506 pages loaded? That’s a figure we can do something with. We can order MDN search results by popularity. We can prioritize performance efforts in Gecko to focus on the most-encountered features. We can view the popularity over time and see when we expect we’ll be able to deprecate and remove the feature.

This is why you’ll more likely encounter use counter data expressed as usage rates.