go-mht

README

go-mht

An MHT (MHTML) parser and writer for Go, built for byte-faithful round trips of Chrome/Blink-generated snapshots.

Why

Chrome's "Save as MHTML" (and ChromeDP's page.CaptureSnapshot) produce MHTML files with specific conventions: a fixed header order, tab-folded header values, quoted-printable HTML bodies, and 76-column base64 for binary parts.

Go's standard library (net/mail + mime/multipart + mime/quotedprintable) can parse these files, but it cannot write them back faithfully: headers get reordered and canonicalized, and quoted-printable content re-encodes differently. If your workflow is parse → modify → write — for example, swapping an image inside a snapshot — the output drifts from the original and can render differently.

go-mht parses with a purpose-built scanner that preserves Blink's conventions. Parsing a Chrome-generated MHT and writing it back out produces a byte-identical file (this is enforced by a golden test).

It also handles a ChromeDP quirk: when an MHT is opened from disk and re-exported, Blink rewrites the root Content-Location to the local file path, which breaks rendering. documents.FixContentLocation restores the original URL.

Install

go get github.com/goodblaster/go-mht

Library usage

Parse and write

doc, err := documents.ParseFile("snapshot.mht")
if err != nil {
    log.Fatal(err)
}

out, _ := os.Create("copy.mht")
defer out.Close()
if err := doc.Write(out); err != nil {
    log.Fatal(err)
}
// copy.mht is byte-identical to snapshot.mht

Work with images

// All image sections.
for _, s := range doc.ImageSections() {
    fmt.Println(s.Headers.Get("Content-Location"))
}

// Decode one image by its Content-Location.
img, format, err := doc.Image("https://example.com/logo.png")

// Or grab the raw bytes.
data, err := doc.ImageBytes("https://example.com/logo.png")

// Replace an image in place, then write the document back out.
section := doc.ImageSection("https://example.com/logo.png")
err = section.ReplaceImage("image/png", "base64", newPNGBytes)

Fix a ChromeDP re-export

location, err := documents.ExtractContentLocation("original.mht")
// ... re-export via ChromeDP, then:
fixed := documents.FixContentLocation(reExportedContent, location)

CLI

go install github.com/goodblaster/go-mht/cmd/mht@latest

mht snapshot.mht                     # parse and write to stdout
mht -o out.mht snapshot.mht          # parse and write to a file
mht -images ./imgs -o out.mht in.mht # also extract all images

Since output is byte-faithful, mht -o out.mht in.mht && cmp in.mht out.mht doubles as a check that a file survives a round trip.

Scope and limitations

• Tuned for Blink-generated MHTML (From: <Saved by Blink>). Files from other producers (e.g. Word, IE) may parse, but byte-identical output is only expected for Blink conventions.
• Supported transfer encodings: quoted-printable (the default when the header is absent) and base64; other encodings are passed through unchanged.
• Line endings are expected to be CRLF, as Blink produces.

License

MIT — see LICENSE.