Can I mix wordpress with regular html/css sites?

If you add WordPress to an existing non-WordPress site, you are best to install it under a separate folder, eg:

example.com/blog

All your WordPress pages/posts would then be referenced under that folder.

Yes, you can. It's the way that most site add a blog into their website.

yes you can but your link will look something like this mydomain.com/your-wp-folder/contact

yes absolutely you can, why not create blank page with wordpress page template?

I wouldn't advice to do so, you can recreate your pages with wordpress, but if you want you can just install WP in the same place as your current website (backup your files), and upload your html files back into your website, and you can combine them without a problem.

Sometimes it is easier to just have a html squeeze page instead of using wordpress. But integrating html pages with wordpress to form one website will give you some work to interlink the menu's of your pages.

Goodluck!

Well. I hesitate to chime in here since the OP's question has been answered so thoroughly in the preceding posts, but I'll give it a shot.

Yes, you can mix-and-match an existing HTML-only site with a WordPress installation. No, the URLs don't have to reflect the WP installation folder ( http// mysite.com/wordpressfolder/post-name ) unless you WANT them to.

There are no SEO advantages to having a WP site instead of an HTML site, unless your HTML is totally balled-up in its coding. SEs don't care WHAT generated the HTML, as long as it is as sematically correct as you can make it.

If you want to run WP from its own directory and still have siteroot-based URLs, this is how you do it: Giving WordPress Its Own Directory « WordPress Codex

The only 'gotcha' here is going to be if your HTML site is truly constructed like the examples you gave: mysite.com/page-1 or mysite.com/another-page. You'll notice the absence of the ".html" extension in the URL snippets. If your site is addressed this way, it is likely because of an .htaccess rewrite rule stripping the extension. If this is true, you'll have problems running WP from its own folder without having the installation folder name in the URL.

By your description, I assume you like the idea of using WordPress for creating websites. However, you don't know what to do with the HTML site. Why not just use one WordPress site and move your existing site to the WordPress platform as well?

Fortunately, WordPress has loads of plugins that allow you to make it do things that it is not supposed to do by default. Moving a HTML website to WordPress is one of them.

Here's one such plugin: WordPress › HTML Import 2 « WordPress Plugins

I would advise you to clone the site on another server location. Install WordPress in the new location's root (main) directory. Then use any plugin to import HTML to WP. Once you are done fixing the duplicate site, use Dupliator plugin (WordPress › Duplicator « WordPress Plugins) to create a clone.

Now point the domain to the new server location if you can. Alternatively, create a backup of your live site. Delete everything and install WordPress (check tutorials on Main Page « WordPress Codex). Use Duplicator to have your new WordPress site with all old HTML content added.

By the way, you can install independent instances of WordPress in subdirectories. So if you want mydomain.com/blog, you can create a subdirectory and install WordPress in it.