Page header/footer in Sans Serif Font body in Serif ?

I want to use for chapter titles/section a Sans Serif font and for the normal text a Serif font.

To be consistent:

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  Should page header/footer then be seen as section text (sans serif) or body (serif). I assume page header/footer is section so sans serif?

  
  


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  Should footnotes then be seen as section text (sans serif) or body (serif). I assume body text so serif?

  
  


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  What does often goes wrong when combining sans serif and serif font? What to take into account? E.g. do you see class A (and which class) of text being typeset in sans serif where you would expect serif? What else?

  
  

Tx