"br encoding" is a compression algorithm invented by Google to speed up transfer of data from websites. Web servers that use compressed data such as brotli, should check that a web browser they are sending a response to supports compressed data. We have observed several websites recently that send brotli-compressed responses even when the web browser does not indicate that it understands brotli, meaning that the website will not work at all for some users.
Fortunately, over 96% of users have web browsers that do support brotli (https://caniuse.com/brotli), however this does leave 3.5% of users who will not be able to use the site.
Sitemorse already supports brotli, which is a commonly used compression technique.