You can have these rules in your root .htaccess:
RewriteEngine On
RewriteBase /
# external redirect from actual URL to pretty one (remove query string)
RewriteCond %{THE_REQUEST} \s/+content\.php\?page=([^\s&]+) [NC]
RewriteRule ^ %1? [R=302,L,NE]
# convert all space (%20) to hyphen
RewriteRule "^(\S*) +(\S* .*)$" $1-$2 [N,NE]
RewriteRule "^(\S*) (\S*)$" $1-$2 [L,R=302,NE]
# rewrite rule to call actual PHP handler
RewriteRule ^([^./]+)\.html$ content.php?page=$1 [L,QSA,NC]
