.htaccess - Open files without extension using .htaccess in subdomain - .htaccess

Question

I downloaded a website, all files are coded in HTML and files do not have any extension.
My root domain is WordPress based.
I want to open all files as HTML using .htaccess - the files are in a subdomain.
I have tried this, think its for root domain only... I need help for subdomain.
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.+)$ /$1.php [L,QSA]

Answer 1

Your question is a bit vague as you've not given any examples of the URLs and file structure involved. However, if the HTML files do not have file extensions then it's quite probable that the Content-Type HTTP response header being sent from the server is either wrong or entirely absent (and reliant on the browser "content sniffing" - which is going to get mixed results).
We need to make some assumptions:
All the HTML files (that do not have file extensions) do not contain a dot anywhere in the file path.
The CSS and JS files do have appropriate file extensions ie. .css and .js, and are already returning the correct mime-type (Content-Type header).
If these too don't have file extensions, then they would need to be contained in specific directories so we can set the appropriate mime-type accordingly. We would also need to create exceptions with our rules in order to avoid conflicts.
So, in order to get the browser to interpret these files as HTML we need to make sure that we are sending the correct mime-type (ie. text/html) in the Content-Type response header. (I assume this is not the case currently.)
For example, using mod_rewrite in the .htaccess file in your subdomain (which I assume is separate from your main domain):
RewriteEngine On
RewriteRule ^([^.]+)?$ - [T=text/html]
However, as noted above, if your static resources (JS, CSS, images, etc.) are also devoid of file extensions then we'll need to add conditions to the above rule and create additional rules for the different file/mime types.
Aside:
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.+)$ /$1.php [L,QSA]
These directives that you posted in the question don't appear to have anything to do with your question, which has only added to the confusion in comments.