Font Fallback

[Aerodlyn]

While what you have isn't wrong, setting multiple fonts and font families allows your site's look-and-feel to remain as consistent as possible across as many platforms as possible.

So instead of:
font-family: Helvetica;

You could do:
font-family: Helvetica, sans-serif;

[aosm25]

Do you suggest that I have to keep entering font-family for each subsection of the page where I used ''flexbox model'' or can I just declare it as part of the html {} ?

[Aerodlyn]

Depends on if you plan on using different fonts. If the entire site uses the same font, then you can just do it in either html or body.