RewriteEngine On

RewriteBase /

RewriteCond %{THE_REQUEST} ^GET\ (.*)\.html\ HTTP
RewriteRule (.*)\.html$ $1 [R=301]

RewriteRule (.*)/index$ $1/ [R=301]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} /$
RewriteRule (.*)/ $1 [R=301]

RewriteCond %{REQUEST_FILENAME}.html -f
RewriteCond %{REQUEST_URI} !/$
RewriteRule (.*) $1\.html [L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule . / [L,R=301]