A BR file is a file compressed with Brotli, an open source data compression algorithm. It contains webpage assets, such as .CSS, .XML, .SVG, and .JS files, compressed in the Brotli Compressed Data Format. BR files are used by web browsers such as Chrome and Firefox to increase page loading speeds.
The Brotli Data Compression Format is designed to replace the Zopfli compression algorithm. The format allows for roughly 20% higher compression ratios than Zopfli.
NOTE: Files compressed by Brotli were originally saved with the ".bro" extension but it was changed to the ".br" extension due to complaints of "bro" being misogynystic and unprofessional.