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Measuring web compatibility

This page tracks the telemetry measurements that the Web Compat team is tracking. See Telemetry for more information.

Bug 1109958 will serve as the meta tracking bug for all telemetry probes.

Q1 probe goals

These are the probes we want to try to land in Q1. Please only add to this list if you're willing to take on the required work.

WAP (WML or XHTMLMP) content type

We not infrequently receive bugs in Bugzilla about WAP content being sent to Fennec and Firefox OS users (see bug 941241, bug 997668, with dependencies, among others.). We intend to gather telemetry data to understand how common this is for Firefox mobile users. Once we understand the frequency and severity of the problem (measured as WAP vs non-WAP content type responses) we can decide how to proceed. Possible decisions include: support re-parsing of WAP content as HTML, showing an error message in the developer console, WONTFIXing related bugs and continuing to do Tech Evangelism as issues arise.

A telemetry probe (HTTP_WAP_CONTENT_TYPE_RECEIVED histogram) was landed in bug 1121559 on January 21, 2015.


We need to gather telemetry on the videos being used in the wild. This directly tells us what features matter for openh264 development and for our own products. It also helps us track uptake use of other codecs such as h265 or proprietary dolby digital audio codecs, for example.

  • Media formats and MIME types
    • HTMLMediaElement.canPlayType()
    • MediaSource.isTypeSupported()
    • MediaSource.addSourceBuffer(DOMString type)
    • ogg, mp4, h264, m3u8/HLS, etc.

For the MSE probes, the question has come up if other sites use mp3 in MSE (other than Google Play). See

  • H264
    • h264 profile and level, measured from canPlayType() (See Bug 1125340).
    • h264 profile and level, measured from decoded SPS (see Bug 1125340)
    • h264 constraint_set flags, measured from decoded SPS (see Bug 1136877)
    • SPS.max_num_ref_frames (see Bug 1136877)

Idea bucket (sanity not guaranteed)


  • `display: table` with `max-width: 100%` on children (see 434230 and 975632 )


  • `Server:` name
  • Redirection sequences
  • Wrong mimetypes for images
  • Wrong mimetypes for JSON



  • Ads before video and transition to video
  • EME required
  • List of video providers (stats for understanding if we need to focus our contacts actions toward some specific people)
  • List of JS libraries used for video players
  • Flash-only video (perhaps already covered in 1108668)
  • Video dimensions
  • Compressed video sample sizes
  • Frame drops
  • Hardware or software decoders
  • Playing videos concurrently
  • Audio speaker configuration e.g. mono, stereo, 5.1
Related information

h264 rec: