December 20, 2016 10:00 am

Introducing Brotli compression in Microsoft Edge

Beginning with EdgeHTML 15.14986, Microsoft Edge supports Brotli as an HTTP content-encoding method. This change will be released to stable builds with the Windows 10 Creator’s Update early next year, but you can preview it now via the Windows Insider Program. With this release, Brotli will be broadly interoperable across browsers, with support in the latest versions of Microsoft Edge, Firefox, and Chrome.

Brotli is a compression format defined in RFC 7932, previously available as part of the WOFF2 font format. When used as an HTTP content-encoding method, Brotli achieves up to 20% better compression ratios with similar compression and decompression speeds (PDF). This ultimately results in substantially reduced page weight for users, improving load times without substantially impacting client-side CPU costs. As compared to existing algorithms, like Deflate, Brotli compression is more efficient in terms of file size and CPU time.

In the current preview release, Microsoft Edge supports Brotli on HTTPS and HTTP connections. In a future preview release, we will update this behavior to only advertise Brotli support on HTTPS connections. Like Chrome, we will continue to decode Brotli content on HTTP connections. Note that in the current preview release, there is a known issue which results in the F12 Developer Tools incorrectly not showing the accept encoding response header. This is tracked as issue 9771399 on

As always, we welcome your feedback on Brotli in Microsoft Edge! Let us know on Twitter or in the comments below if you have any questions or issues. We’re very much looking forward to making the web just a little bit lighter with Brotli!

― Rob Trace, Senior Program Manager, Microsoft Edge

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  1. This: “As compared to existing algorithms, like Deflate, decompression with Brotli is much less expensive without sacrificing compression ratios.” is a mischaracterization.

    Decompressing Brotli remains *fast*, but it’s not “much less expensive” than Deflate. The point is that Brotli achieves a significantly improved compression ratio vs. Deflate with only a modest hit to compression speed at the highest compression levels. By way of example, Brotli is much more efficient than the Deflate-generating Zopfli compressor, yielding both size and CPU time improvements.

    Learn more about Brotli here:

    • To quote Kenji Baheux:
      ‘Intermediaries (or “middle boxes”) refers to companies/infra/software meddling with the data transfer between you (the user) and the webserver.

      One example from SDCH that was mentioned to me (the name of the company is not relevant to the discussion so I’m hiding it):

      “The most extreme case of middle box was Company AcmeTelecom (fictitious name but true story), that tried to make things better when faced with unknown content encodings, by doing the following things:

      a) Remove the unrecognized content encoding
      b) Pass the (already compressed!) content through another gzip encoding pass
      c) Claim that the content was merely encoded as gzip”‘