Hey @buzut, if you have Pretty URLs enabled, people will not get to the .html versions of your pages from your site since we rewrite the HTML of your site to strip the .html extension away. They would only get to those pages if they type them in directly. You could try adding a 301 redirect for every file ending in .html, but it seems like a lot of work for minimal benefit. For each article or post, you’d have to do something like this, and even then you may hit an infinite loop problem since you’re redirecting to the same page:
/article.html /article 301!
This is also not something our redirects would parse:
/*.html /:splat 301!
Since you’re thinking about SEO, I did want to share this post by our CTO about a canonical link header we rolled out for when you enable a custom domain on your site- might be of interest to you 
:
Hope this helps! Please let us know if we can answer any other questions on this.