1. httpheaders can be customize on each page including title,keywords - in addition the system can be extended by custom modules and httpmodules and skinobjects - all of which can further modify and manage these elements.

2. dnn is html5 compliant - not sure what you mean by custom HTML tags beyond this - html is rendered by the client browser.   If you maybe mean custom tokens/macros - there is an extensive dnn tokens in the html module that can be used to reference core data - and there are some 3rd party macro replacement modules that extend this capability in a variety of ways depending on your needs.

3. anchor tags are pretty much depreciated in html5 but there is support intelligent javascript scrolling thru jquery type optimizations.

4. dnn urls are based initially on the page layout structure defined by the user - BUT in addition - the advanced url system can be used to dynamically define any url structure you require. 

5. you are free to layout and structure you pages in any way you require - you can control on a page by page basis what metatabs are utilized.

6. the advanced url provider lets you define any url appearance you desire for each individual page.

7. again see advance url provider capabilites regarding 301 - but in addition it is also possibly to extend the system with custom httpmodules and httphandlers to meet any other unique needs.

8. the HTML module and media modules provide a number of options for image management and alt tags - and there are numerous 3rd party modules that extend these features further.

9. you are free to use dnn however you demand - it is a content delivery platform and management system that serves up native html and css to web browsers .  you define any css structures you need thru skins that again as simply html/css files.

10. dnn profiles a wide variety of caching options at both page level and module/component levels - these are customizable on a module/page basis.

11. dnn generates xml sitemaps dynamically from its internal database tables and structures as needed. 

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Now having said all this - I would strongly suggest that you actually take a look at how dnn works by downloading and installing a copy - or by taking advantage of the free online trial hosting platform - some of the concepts you are asking about would suggest that maybe you dont really have a strong understanding on how a CMS like DNN works.

DNN is a framework that you can utilize in a wide variety of flexible ways.

Westa