The code below has work for me in the past 🙂
function add_to_htaccess( $rules ) {
$content = <<<EOD
# Enable Vary: Accept-Encoding Start
<IfModule mod_headers.c>
<FilesMatch "\.(js|css|xml|gz)$">
Header append Vary: Accept-Encoding
</FilesMatch>
</IfModule>
EOD;
return $content . $rules;
}
add_filter('mod_rewrite_rules', 'add_to_htaccess');