Examples of websites using
Helvetica
(Helvetica is a sans serif font designed by Max Miedinger, Linotype Design Studio, and released by Linotype in 1957. It is available in a wide range of styles and weights)
with
Avenir
(Avenir is a sans-serif font designed by Adrian Frutiger and Nadine Chahine, released by Linotype in 1988. It features clean and modern letterforms and is available in multiple weights and styles)