ErrorDocument 404 /index.html

Options -Indexes
DirectoryIndex index.php index.html

<IfModule mod_rewrite.c>
RewriteEngine On

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

RewriteCond %{REQUEST_FILENAME} -d
RewriteCond %{REQUEST_FILENAME}/index.php !-f
RewriteCond %{REQUEST_FILENAME}/index.html !-f
RewriteRule ^(.+)/$ /index.html [L,R=301]
</IfModule>