Not having an .html file extension - SEO benefits over an extensionless directory-style url
On the .html file extension:
A file extension makes any keyword-rich naming appear human-generated rather than computer-generated, since html files are typically static files. They also argue that a page named “/awesome-page.html” is considered one level higher than one named “/awesome-page/” and that higher level pages get more indexing-juice.
On the other side:
It doesn’t matter. It’s likely that Google does not really assign more weight to a page with an .html file extension, as extensions can easily be generated dynamically. Surely, Google or other search engines aren’t so naive as to make an assumption such as, “If the page has an .html extension it must be static”. Secondly, while it is true that pages higher up in the directory structure of a site get more weight in indexing, it’s also true that this is considered in relation only to the site itself. At that point, the argument becomes moot, since whether the site structure starts at level 0 or level 1, all sub-levels will line up equally. Lastly, and perhaps most importantly, the W3C recommends directory-style urls over extensions (under the heading, “What to leave out”).