SYNTAX QUERY - HTML and CSS responsive navigation bar using FlexBox
Here are a few thoughts:


The universal selector to reset the margin and padding is slow and unnecessary in the case since none of the elements you're using have default margin or padding.
The use of hover by itself is not mobile friendly, and if the user expects to click Home to go to the homepage, a more robust design would be better. For example:



  It is a good idea to double up focus and hover styles, so your users get that visual clue that a control will do something when activated, whether they are using mouse or keyboard
  For situations when the parent menu item needs to carry out a function, such as linking to a web page, a separate button can be added to the parent item, to open and close the submenu. This button can also act as a visual indicator for the presence of a submenu.



For accessibility, add aria-role=menubar to the ul and aria-role=menuitem to each anchor tag.


References


Fly-out Menus • Menus • WAI Web Accessibility Tutorials
Navigation Menubar Example | WAI-ARIA Authoring Practices 1.1
Handling common accessibility problems - Learn web development | MDN