See this section of the .htaccess file:
## These directives are only enabled if the Apache mod_headers module is enabled.
## This section will check if a .gz file exists and if so will stream it
## directly or fallback to gzip any asset on the fly
## If your site starts to look strange after enabling this, and you see
## ERR_CONTENT_DECODING_FAILED in your browser console network tab,
## then your server is already gzipping css and js files and you don't need this
## block enabled in your .htaccess
<IfModule mod_headers.c>
...
</IfModule>
If present, comment out or remove.
