This section is related to the article, Leverage Browser Caching, so be sure to read that first.
In addition to the codes to leverage browser caching, the Optimize .htaccess button also adds codes to the .htaccess file at the root of your site to compress static files using gzip. The gzip codes will look like this:
<IfModule mod_deflate.c> AddOutputFilterByType DEFLATE text/html AddOutputFilterByType DEFLATE text/css AddOutputFilterByType DEFLATE text/javascript AddOutputFilterByType DEFLATE text/xml AddOutputFilterByType DEFLATE text/plain AddOutputFilterByType DEFLATE image/x-icon AddOutputFilterByType DEFLATE image/svg+xml AddOutputFilterByType DEFLATE application/rss+xml AddOutputFilterByType DEFLATE application/javascript AddOutputFilterByType DEFLATE application/x-javascript AddOutputFilterByType DEFLATE application/xml AddOutputFilterByType DEFLATE application/xhtml+xml AddOutputFilterByType DEFLATE application/font AddOutputFilterByType DEFLATE application/font-truetype AddOutputFilterByType DEFLATE application/font-ttf AddOutputFilterByType DEFLATE application/font-otf AddOutputFilterByType DEFLATE application/font-opentype AddOutputFilterByType DEFLATE application/font-woff AddOutputFilterByType DEFLATE application/font-woff2 AddOutputFilterByType DEFLATE application/vnd.ms-fontobject AddOutputFilterByType DEFLATE font/ttf AddOutputFilterByType DEFLATE font/otf AddOutputFilterByType DEFLATE font/opentype AddOutputFilterByType DEFLATE font/woff AddOutputFilterByType DEFLATE font/woff2 # For Olders Browsers Which Can't Handle Compression BrowserMatch ^Mozilla/4 gzip-only-text/html BrowserMatch ^Mozilla/4\.0[678] no-gzip BrowserMatch \bMSIE !no-gzip !gzip-only-text/html </IfModule>
As in the case for leverage browser caching, these codes don't affect files loaded dynamically from external servers. If you're seeing local static files being loaded that are not compressed, check with your host to see if mod_deflate is installed and enabled on the server.
If the static files that require compression are the combined css or js files generated by the plugin, you can select another option for the Combine files delivery setting in the Combine CSS and javascript files feature on the Basic Options tab. The settings other than Static CSS and js files uses PHP files to deliver the combined css or js file and they will be gzip compressed using PHP for all automatic settings above Basic.