Right now, mailto: urls only support html-body to force mail compose to open up in HTML mode. There not a way to force to open up in PlainText mode via the url. Supporting the following: Content-Type=text/plain Content-Type=text/html would allow the url to control the mode of mail compose opens up in. 'html-body' is not a standard, but Content-Type isn't as well. However, Content-Type more closely aligns with HTML standards than 'html-body'.

Sean Su: Could you post a URL with the "html-body" argument? I don't know what the format is for that, and a Google search didn't help. Is there an RFC that supports this Content-Type idea?

RFC 2368 recommends against allowing mailto links to set Content-Type. See "7. Security Considerations" in ftp://ftp.rfc-editor.org/in-notes/rfc2368.txt

(In reply to J Luh from comment #2) > RFC 2368 recommends against allowing mailto links to set Content-Type. See > "7. > Security Considerations" in ftp://ftp.rfc-editor.org/in-notes/rfc2368.txt https://www.ietf.org/rfc/rfc2368.txt Let's close as WONTFIX, then.