When did HTML email become common?

HTML email became mainstream in the late 1990s with the rise of commercial email clients like Outlook and Hotmail (1996). Hotmail was a pioneer of webmail, allowing anyone to access email from anywhere rather than being tied to an ISP or a specific computer.

It allowed for formatted layouts, fonts, and visuals, turning the plain digital letter into a fully designed message.

Its rise sparked a "holy war" among developers. Purists argued that email should remain plain text for accessibility and security. Marketers and designers pushed the visual frontier forward, a tension that continues even today.

Netscape Navigator 2.0 in 1996 was one of the first browsers to render HTML email, and Microsoft Outlook 97 brought HTML composition to the masses, cementing its place in email culture.

Marketers quickly embraced it, and the inbox transformed from a simple mailbox into a colorful, interactive marketplace.