2030898 - Add MHTML (RFC 2557) export support to "Save Page As"

Status: ASSIGNED

(Core :: DOM: Serializers, enhancement)

Description

[Jonathan Kingston [:jkt] he/him]

Add an MHTML option to the Save Page dialog so users can save a complete web page as a single .mhtml file, interoperable with Chromium and other tools. This is the write half of bug 40873, split out so it can land independently of read support.

Comment 1

[Jonathan Kingston [:jkt] he/him]

Attachment: WIP: Bug 2030898 - Add MHTML import support (PoC, import-only).

Lifted from jkt/40873 tip (969cc8065d5f) onto main without export:
no nsMHTMLPersist, no WebBrowserPersist MHTML save path, no Save Page UI.
Browser tests that depended on export or the reftest generator were removed
or adjusted; add a checked-in mhtml-basic-rendering.mhtml for the basic reftest.

MHTML deltas are applied on top of current main for Document, nsDocShell,
LoadInfo, dom/security/moz.build, netwerk/protocol/moz.build, and
ActorManagerParent (avoid reverting unrelated main changes).