RewriteEngine On

RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME}.html -f
RewriteRule ^([^/]+)$ $1.html [L]

RewriteCond %{THE_REQUEST} \s/+(.+?)\.html[\s?] [NC]
RewriteRule ^ /%1? [R=301,L]