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The mdn-browser-compat-data JSON schema

This document helps you to understand how mdn-browser-compat-data is organized and structured.

Where to find compat data

The folder structure

Compatibility data is organized in top-level directories for each broad area covered: for example, "http", "javascript", "webextensions". Inside each of these directories is one or more JSON file containing the compatibility data.

  • api/ contains data for each Web API interface.

  • css/ contains data for CSS properties, selectors and at-rules.

  • html/ contains data for HTML elements, attributes and global attributes.

  • http/ contains data for HTTP headers, statuses and methods.

  • javascript/ contains data for JavaScript built-in Objects, statement, operators and or other ECMAScript language features.

  • webextensions/ contains data for WebExtensions JavaScript APIs and manifest keys.

File and folder breakdown

The JSON files contain feature identifiers, which are relevant for accessing the data. Except for the top-level directories, the file and sub folder hierarchies aren't of any meaning for the exports. Compatibility data can be stored in a single large file or might be divided in smaller files and put into sub folders.

Understanding the schema

Feature hierarchies

Each feature is identified by a unique hierarchy of strings. E.g the text-align property is identified by css.properties.text-align.

In the JSON file it looks like this:

{
  "css": {
    "properties": {
      "text-align": {
        "__compat": {},
        "start": {
          "__compat": {}
        },
        "end": {
          "__compat": {}
        }
      },
    }
  }
}

Each feature is uniquely accessible, independently of the file it is defined in.

The hierarchy of identifiers is not defined by the schema and is a convention of the project using the schema.

Features

A feature is described by an identifier containing the __compat property. In other words, identifiers without __compat aren't necessarily features, but help to nest the features properly.

When an identifier has a __compat block, it represents its basic support, indicating that a minimal implementation of a functionality is included. What it represents exactly depends of the evolution of the feature over time, both in terms of specifications and of browser support.

Sub-features

To add a sub-feature, a new identifier is added below the main feature at the level of a __compat object (see the sub-features "start" and "end" above). The same could be done for sub-sub-features. There is no depth limit.

The __compat object

The __compat object consists of the following:

  • A mandatory support property for compat information. An object listing the compatibility information for each browser (see below).

  • An optional description property to describe the feature. A string containing a human-readable description of the feature. It is intended to be used as a caption or title and should be kept short. The <code> and <a> HTML elements can be used.

  • An optional status property for status information. An object containing information about the stability of the feature: Is it a functionality that is standard? Is it stable? Has it been deprecated and shouldn't be used anymore (see below).

  • An optional mdn_url property which points to an MDN reference page documenting the feature. It needs to be a valid URL.

The support object

Each __compat object contains support information. For each browser identifier, it contains a support_statement object with information about versions, prefixes or alternate names, as well as notes.

Browser identifiers

The currently accepted browser identifiers should be declared in the following order:

  • webview_android, Webview, the former stock browser on Android,
  • chrome, Google Chrome (on desktops),
  • chrome_android, Google Chrome (on Android),
  • edge, MS Edge (on Windows),
  • edge_mobile, MS Edge, the mobile version,
  • firefox, Mozilla Firefox (on desktops),
  • firefox_android, Firefox for Android, sometimes nicknamed Fennec,
  • ie_mobile, Microsoft Internet Explorer, the mobile version,
  • ie, Microsoft Internet Explorer (discontinued),
  • nodejs Node.js JavaScript runtime built on Chrome's V8 JavaScript engine,
  • opera, the Opera browser (desktop), based on Blink since Opera 15,
  • opera_android, the Opera browser (Android version),
  • safari, Apple Safari, on Mac OS,
  • safari_ios, Apple Safari, on iOS,
  • samsunginternet_android, the Samsung Internet browser (Android version).

No browser identifier is mandatory.

The support_statement object

The support_statement object describes the support provided by a single browser type for the given subfeature. It is an array of simple_support_statement objects, but if there is only one of them, the array must be omitted.

Example of a support compat object (with an array_support_statement containing 2 entries):

"support": {
  "firefox": [
    {
      "version_added": "6"
    },
    {
      "prefix": "-moz-",
      "version_added": "3.5",
      "version_removed": "9"
    }
  ]
}

Example of a support compat object (with 1 entry, array omitted):

"support": {
  "ie": { "version_added": "6.0" }
}

Compat data in support statements

The simple_support_statement object is the core object containing the compatibility information for a browser. It consist of the following properties:

version_added

This is the only mandatory property and it contains a string with the version number indicating when a sub-feature has been added (and is therefore supported). The Boolean values indicate that a sub-feature is supported (true, with the additional meaning that it is unknown in which version support was added) or not supported (false). A value of null indicates that support information is entirely unknown. Examples:

  • Support from version 3.5 (inclusive):
{
 "version_added": "3.5"
}
  • Supported, but version unknown:
{
  "version_added": true
}
  • No support:
{
  "version_added": false
}
  • Support unknown (default value, if browser omitted):
{
  "version_added" : null
}

version_removed

Contains a string with the version number the sub-feature is removed in. It may also be a Boolean value of (true or false), or the null value.

Default values:

  • If version_added is set to true, false, or a string, version_removed defaults to false.
  • if version_added is set to null, the default value of version_removed is also null.

Examples:

  • Removed in version 10 (added in 3.5):
{
  "version_added": "3.5",
  "version_removed": "10"
}
  • Not removed (default if version_added is not null):
{
  "version_added": "3.5",
  "version_removed": false
}

prefix

A prefix to add to the sub-feature name (defaults to empty string). Note that leading and trailing - must be included. Example:

  • Prefixed sub-feature:
{
  "prefix": "-moz-",
  "version_added": "3.5"
}

alternative_name

In some cases features are named entirely differently and not just prefixed. Example:

  • Prefixed version had a different capitalization
{
  "alternative_name": "mozRequestFullScreen",
  "version_added": "true",
  "version_removed": "9.0"
}

flag

An optional object indicating what kind of flags must be set for this feature to work. It consists of three properties:

  • type (mandatory): an enum that indicates the flag type:
    • preference a flag the user can set (like in about:config in Firefox).
    • compile_flag a flag to be set before compiling the browser.
    • runtime_flag a flag to be set before starting the browser.
  • name (mandatory): a string representing the flag or preference to modify.
  • value_to_set (optional): representing the actual value to set the flag to. It is a string, that may be converted to the right type (that is true or false for Boolean value, or 4 for an integer value). It doesn't need to be enclosed in <code> tags.
{
  "version_added": true,
  "flag": {
    "type": "preference",
    "name": "browser.flag.name",
    "value_to_set": "true"
  }
}

partial_implementation

A boolean value indicating whether or not the implementation of the sub-feature follows the current specification close enough to not create major interoperability problems. It defaults to false (no interoperability problem expected). If set to true, it is recommended to add a note indicating how it diverges from the standard (implements an old version of the standard, for example).

notes

An array of zero or more strings containing additional information. If there is only one entry in the array, the array can be a just a string. Example:

  • Indicating a restriction:
{
  "version_added": "3.5",
  "notes": [
    "Does not work on ::first-letter pseudo-elements.",
    "Has not been updated to the latest specification, see <a href='https://bugzil.la/1234567'>bug 1234567</a>."
  ]
}

The <code> and <a> HTML elements can be used.

Status information

The status property informs about stability of the feature. It is an optional object named status and has three mandatory properties:

  • experimental: a boolean value that indicates this functionality is intended to be an addition to the Web platform. Some features are added to conduct tests. Set to false, it means the functionality is mature, and no significant incompatible changes are expected in the future.
  • standard_track: a boolean value indicating if the feature is part of an active specification or specification process.
  • deprecated: a boolean value that indicates if the feature is no longer recommended. It might be removed in the future or might only be kept for compatibility purposes. Avoid using this functionality.
"__compat": {
  "status": {
    "experimental": true,
    "standard_track": true,
    "deprecated": false
  }
}

Localization

We are planning localize some of this data (e.g. notes, descriptions). At this point we haven't decided how and when we are going to do that. See issue 114 for more information.