The point of SGML/XML/HTML was to allow device independent transfer of information between people, since an author cannot rely on a reader having the same device or software, which is still true.
Presentation was and is left to the reading device, but as authors we now can give hints about how a document should be presented, but the reading device or the reader can still disregard these hints. However, what happens when no hints are given? The reading device makes a decision, and this might be horribly wrong if your design stray from common browser defaults.
We now have a split between semantics and presentation, i.e. what is a paragraph - HTML - and what is red paragraph - CSS. I will highly recommend to take control of all HTML elements in your CSS file, i.e. give all of them a default hint.
You can quickly create a default CSS file by looking at and including hints for all the elements from
https://www.w3.org/community/webed/wiki/HTML/Elements . You then have a template CSS file, which you can tune to each project in no time.