The web design focuses on visualizing the structure of the data being presented. It clearly signals the importance of any given element for the current page and supports the reader to easily navigate the site to the desired destination.

Things that are interactable and cause an action at the current page, should have inset shadows (in order to imitate a 3D button), be round and bold. Things that are used for navigation should have outset shadows (and therefore be flat), have corners and be bold. Any kind of interactable things should use sans serif fonts and non-interactive things should have serif fonts.

Non-text visual elements like additional lines and boxes, can be used in order to visualize the connections, groupings and structures of things, but they create noise, that distract a lot. Prefer to do this only, when needed from the content's and functionalitie's perspective and not in order to make something nice. Try to use text and character symbols (UTF-8) first, in order to visualize the structure of something. This is not something, that is a hard rule, but be wary on changing design decisions regarding this aspect repeatedly. See design of no-code editor, where this aspect in design is hard to finalize.