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Web / FAQ / Content encoding warnings

Why does WebViewer give a warning about Content-Encoding when loading from my server?

WebViewer contains certain large files that are already compressed using brotli (abbreviated br) or gzip encoding. As the warning suggests for ideal performance your server should be adjusted to serve these files with the HTTP Content-Encoding header. (see for more details on Content-Encoding).

The main reason for this is that with this header the browser can quickly decompress these files using native code. Note that WebViewer can still function by decompressing these files in JavaScript, but this may have an impact on loading speed.

General Steps

Note that in order to serve files with Content-Encoding: br your site must use HTTPS rather than HTTP. This is due to behaviour in certain browsers (in particular Chrome) that leads them to reject brotli encoding served over HTTP.

The goal is to serve files within WebViewer containing ".gz." in their file name with "Content-Encoding: gzip" and files containing ".br." in their file name with "Content-Encoding: br".

For Apache Servers

WebViewer includes a .htaccess file in the core directory (eg. WebViewer/lib/core) that implements this logic for you.

Please make sure the .htaccess file got copied over to your server, and that .htaccess files are enabled. Please also make sure that your server has mod_headers enabled.


  1. Make sure the URL rewrite module is installed and active. You can download the module here.
  2. Download this web.config file, and place it in the core directory (eg. WebViewer/lib/core).

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