The big idea with ProtonMail is end-to-end encryption, so that Proton can't read your email. Among the things such a guarantee makes impossible is a simple HTML view, because it would require the Proton-controlled server to decrypt your email before generating the HTML page.

It also makes traditional protocols like SMTP, POP and IMAP unusable. For that Proton provides a local gateway so that you can use your favorite email client, but Proton servers can't offer such a feature without breaking E2EE.