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There is no set definition or standard. Clean/Pretty is a concept to say that the URL should not have unnecessary and hard to read parts. My interpretation is that it should not be 'dirtied' by extra data or details irrelevant to visitors. The CNAME can be part of the domain and does not necessarily make URLs not clean but it should be predictable.
Generally ...
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The Cname www if it is set to be part of the base URL is OK. It depends if the website uses www or non www. But it should be consistent to avoid duplicate contents. So it would still be a clean URL with or without. Consistency is a must for SEO point of view.
Yes clean url should not contain query strings and extensions like .html, .php, etc. I remember ...
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On Apache 2.4 you can convert the URL-path to lowercase in .htaccess (only) with an Apache Expression in a RewriteCond (mod_rewrite) directive. No need to define a RewriteMap in the server config (or write a plethora of rules to replace each letter one by one).
For example:
RewriteEngine On
RewriteCond expr "tolower(%{REQUEST_URI}) =~ /(.*)/"
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