# What Is MHTML (.mht, .mhtml)? — Single-File Web Pages, and Where They Fail
Canonical: https://commareports.com/glossary/mhtml
Published: 2026-11-12
> MHTML is a web page and all its resources packed into one MIME multipart file (.mht or .mhtml). How it works, which browsers open it, why it won't open on phones, from a server or in Firefox, and the portable alternative.
# MHTML
**MHTML (MIME HTML) is a format for saving a web page and every resource
it needs — stylesheets, images, scripts, fonts — as a single file, using
the MIME `multipart/related` structure from email.** It is defined in
RFC 2557 (1999). Files end in `.mht` or `.mhtml`.
## How it works
An MHTML file is literally an email-shaped message:
```
From:
Subject: Test report
MIME-Version: 1.0
Content-Type: multipart/related; type="text/html"; boundary="----MultipartBoundary--abc"
------MultipartBoundary--abc
Content-Type: text/html
Content-Location: https://example.com/report/
……
------MultipartBoundary--abc
Content-Type: text/css
Content-Location: https://example.com/report/style.css
body { font-family: system-ui }
```
Each part carries a `Content-Location`. When the browser renders the HTML
part and meets `style.css`, it resolves the URL and serves the matching
part from inside the archive instead of the network. Binary parts are
base64-encoded, which costs about a third more bytes — the same tax as a
[data URI](/glossary/data-uri).
## Who makes it
- **Chrome and Edge:** Save as → _Webpage, Single File_ (`.mhtml`).
- **Internet Explorer** originated it as _Web Archive, single file_
(`.mht`).
- **Microsoft Word and Excel:** _Single File Web Page_ (`.mht`) — one
reason old intranet reports still arrive in this format.
- **Safari** does not; its single-file format is `.webarchive`, a property
list that only Safari reads.
## Where it fails
MHTML solves "keep it as one file" and then works almost nowhere else:
| Opened from | Result |
| ---------------------------------- | ----------------------------------- |
| Chrome / Edge, local file, desktop | Renders |
| Firefox | Not supported without an extension |
| Safari | Not supported |
| iPhone or Android | No app renders it |
| Served from a website over HTTPS | Downloaded or refused, not rendered |
| Email attachment preview | Shown as text, or as a download |
The server case is deliberate. An archive can contain parts whose
`Content-Location` claims any URL, so rendering a downloaded MHTML as if
it were those pages would be a spoofing hole. Chromium only renders MHTML
from a local file.
That rules it out as a way to share a report: you can't host it, phones
can't open it, and half of desktop browsers can't either.
## The portable alternative: self-contained HTML
The goal MHTML is chasing — one file, nothing missing — is reached more
portably by a [self-contained HTML file](/glossary/self-contained-html):
CSS in `