It depends on your font. Choose one that supports all the Greek characters a good policy. Go to Google Fonts to find a good one.
Since a while back, last century, around Netscape Navigator 3.0 let's say there was basically three fonts I think:
Times New Roman, seri
At some point later Verdana sans became widespread (late 1990's).
Then nothing...... in 1999 Shigetaka Kurita invented Emoiji for NTT DoCoMo in Japan.
Desperate designers began using flash and "Swiffobject" text replacement which sucked.
It was getting pretty messy then Google thankfully launched G Fonts library in 2010. You can see on their analytics page, the most popular font today is Open Sans with 4 trillion views in a year wow. https://fonts.google.com/analytics