RewriteEngine On

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

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