first, security: the web stack is almost impossible to implement securely. there will be vulnerabilities, which will be mercilessly exploited. second, privacy: tracking pixels and other spyware are everywhere in emails. third, accessibility: plaintext emails are a piece of cake for screen readers and braille displays, while HTML emails are a very mixed bag; and plaintext is universal; every email client, no matter how basic or esoteric, is able to display it.
first, security: the web stack is almost impossible to implement securely. there will be vulnerabilities, which will be mercilessly exploited. second, privacy: tracking pixels and other spyware are everywhere in emails. third, accessibility: plaintext emails are a piece of cake for screen readers and braille displays, while HTML emails are a very mixed bag; and plaintext is universal; every email client, no matter how basic or esoteric, is able to display it.
Doesn’t blocking external content in emails (like images) prevent the spyware from working?
yes, but there are still the problems of security and accessibility. blocking external content is a band-aid.