multipartalternative

Multipart/alternative is a MIME container used in email and other internet protocols to deliver the same content in multiple representations. The most common pairing is a plain text version and an HTML version of the message body, allowing recipients to view content regardless of client capabilities.

In a multipart/alternative payload, each part is an alternative representation of the same information. The parts are ordered from least capable to most capable, with the last part typically being the preferred or richest format. The overall message uses a Content-Type header such as multipart/alternative; boundary=... and the parts are separated by boundary delimiters. Each part carries its own headers, like Content-Type (text/plain or text/html) and Content-Transfer-Encoding.

Behavior: user agents should present the last part that they can render. If a client cannot render HTML, it will fall back to the plain text portion. If HTML is rendered, the user may still access or view the plain text as a fallback.

Considerations: when composing, ensure the plain text version is complete and accessible, avoid duplicating content in conflicting ways, and keep styling within the HTML part self-contained to improve reliability and security. The multipart/alternative mechanism is not intended for content that requires synchronous updates or complex dependencies.

Example: an email message declares Content-Type: multipart/alternative; boundary="abc". It includes a first part with Content-Type: text/plain; charset=UTF-8 containing a plain-text version, followed by a second part with Content-Type: text/html; charset=UTF-8 containing a formatted HTML version.