RewriteEngine On

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

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