HTML is the markup of the web, with native headings and tables. ODT is OpenDocument text, the native format of LibreOffice Writer. pandoc sits between the two, and this page says exactly what happens to an HTML file on the way to becoming an ODT one.
What runs when an HTML file becomes ODT
pandoc in two passes, on a machine we rent and watch. Your HTML file is uploaded once, pandoc runs once, the ODT comes back, and neither file is kept. HTML to ODT is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the HTML into the ODT
The heading hierarchy. pandoc parses the HTML file into its own document tree and writes that tree out as ODT, so a level two heading is still a level two heading and not merely bold text.
Tables, rebuilt in ODT syntax rather than screenshotted. That is the difference between a table a machine can read and a picture of one, and it is the whole reason to send an HTML file through pandoc rather than through a printer.
Lists from the HTML file, nesting included, plus code blocks, block quotes and links, all re-expressed in ODT.
Images: pandoc extracts them out of the HTML file into a working directory and re-embeds them in the ODT, so nothing ends up pointing at a file that no longer exists.
What HTML to ODT costs you
The presentation layer. pandoc does not translate the HTML file into ODT, it rebuilds it from its own document tree, so custom styles, page setup, headers and footers do not cross over. The structure does, and that is what the tree is made of.
Where HTML and ODT files come from
ODT. LibreOffice Writer writes it natively. Like ODS it is ISO/IEC 26300, and it turns up wherever a public body or a policy asked for an open format rather than a vendor one.
If a model is going to read the ODT
Structure is what survives from the HTML file, and structure is what a model needs. HTML in, ODT out, with the heading tree intact instead of flattened into one long paragraph.
HTML to ODT, measured rather than promised
Running HTML to ODT against the real engine with a real file, pandoc wrote 7,629 bytes of ODT in 0.1 seconds, machine otherwise idle. That is one HTML file on one day and not an average, which is why the number is given together with the file that produced it.
The HTML to ODT verdict was reached by a pandoc round trip, meaning the output was read back by pandoc and had to still contain a witness word planted in the source, which proves the content travelled and not merely the container. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the ODT file has anything inside it.
The same HTML file, sent somewhere else
Other outputs the probe measured out of an HTML file, so the cost of choosing ODT can be read against something. Sizes do not compare across engines, because each family was probed with its own HTML witness file.
HTML to PDF: 25,178 bytes in 1.9 seconds, verified by a format signature.
HTML to MD: 371 bytes in under a tenth of a second, verified by a pandoc round trip.
HTML to DOCX: 10,277 bytes in 0.1 seconds, verified by a pandoc round trip.
HTML to EPUB: 5,027 bytes in 0.1 seconds, verified by a pandoc round trip.
HTML to TXT: 270 bytes in under a tenth of a second, verified by reading the text back.
HTML to RTF: 775 bytes in under a tenth of a second, verified by a pandoc round trip.
Other ways into ODT, and what they measured
Among the published routes into ODT, HTML is the fourth largest output of the 8 measured. The witness files differ, so this ranks the probe run and not your document.
JSON to ODT: 7,537 bytes in 0.1 seconds.
MD to ODT: 7,537 bytes in 0.1 seconds.
TXT to ODT: 6,941 bytes in 0.1 seconds.
DOCX to ODT: 12,806 bytes in 1.2 seconds.
RTF to ODT: 12,670 bytes in 1.2 seconds.
What we will not pretend about HTML to ODT
An HTML file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An HTML to ODT run that passes 60 seconds is killed, and the pandoc process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
32 of the 935 pairs probed in the family that serves HTML to ODT failed, and this pair is not one of them. They fail for reasons worth knowing: a writer that cannot carry what the document is made of, or output the engine could not read back. They are counted here rather than hidden, because a pair that fails quietly is worse than one that fails loudly.
pandoc never writes the ODT through a TeX engine here, and it never writes PDF at all: the eleven engines it would need are not on this machine and a TeX distribution weighs several gigabytes. Anyone who wants a PDF out of an HTML file goes through LibreOffice or calibre, both of which genuinely can.
One HTML file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
HTML to ODT: what people ask
What actually converts my HTML file to ODT?
pandoc does it, in two passes, on a machine we rent and watch. Not a browser trick and not somebody else service: the HTML file is uploaded once, pandoc runs once, the ODT comes back, and neither file is kept afterwards.
How long does HTML to ODT take?
On the file the probe used, pandoc took 0.1 seconds and wrote 7,629 bytes of ODT. That is one real measurement on one real HTML file, not an average and not a promise about yours: a larger HTML takes longer, and past 60 seconds the run is stopped.
What do I lose going from HTML to ODT?
The one to know about first: The presentation layer. pandoc does not translate the HTML file into ODT, it rebuilds it from its own document tree, so custom styles, page setup, headers and footers do not cross over. The structure does, and that is what the tree is made of.
Are the tables in my HTML file still tables in the ODT?
Yes. pandoc reads the HTML tables into its own document tree and writes them back out in ODT syntax, so what you get is a table a machine can parse rather than a picture of one.
Is HTML to ODT free?
There is a free allowance every month, and one HTML to ODT conversion costs half a credit against it. When the allowance runs out the tool says so and stops, rather than quietly handing you a worse ODT.