DirectoryIndex index.html

RewriteEngine On

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^\.]+)$ $1.html [NC,L]
RewriteCond %{THE_REQUEST} /index.html HTTP [NC]
RewriteRule (.*)index.html$ /$1 [R=301,L]