DOC is the pre-2007 Word binary, which most modern tools open reluctantly. HTML is the markup of the web, with native headings and tables. LibreOffice sits between the two, and this page says exactly what happens to a DOC file on the way to becoming an HTML one.
What runs when a DOC file becomes HTML
LibreOffice 24.2, on a machine we rent and watch. Your DOC file is uploaded once, LibreOffice runs once, the HTML comes back, and neither file is kept. DOC to HTML 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 DOC into the HTML
The text, as text in a web page: no download needed to read it, and anything that can read HTML can read it.
Heading levels, written as real HTML headings.
Tables, written as HTML tables rather than as aligned spaces.
What DOC to HTML costs you
Page geometry. HTML has no pages, so margins, page breaks, headers and footers have nowhere to go. This pair turns a document into a web page, not into a picture of one.
Fonts, unless the reader happens to have them. The HTML names them and the browser substitutes what it has.
Where DOC and HTML files come from
DOC. Word wrote it before 2007. It is an OLE2 compound binary rather than a zip of XML, and that difference is why modern parsers, mail filters and document pipelines increasingly refuse it outright instead of half-reading it.
If a model is going to read the HTML
Structure is what survives from the DOC file, and structure is what a model needs. DOC in, HTML out, with the heading tree intact instead of flattened into one long paragraph.
DOC to HTML, measured rather than promised
Running DOC to HTML against the real engine with a real file, LibreOffice wrote 1,626 bytes of HTML in 1.1 seconds, machine otherwise idle. That is one DOC file on one day and not an average, which is why the number is given together with the file that produced it.
The DOC to HTML verdict was reached by reading the internal type and finding a witness word, meaning the output was read back and recognised. 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 HTML file has anything inside it.
The same DOC file, sent somewhere else
Other outputs the probe measured out of a DOC file, so the cost of choosing HTML can be read against something. Sizes do not compare across engines, because each family was probed with its own DOC witness file.
DOC to PDF: 11,917 bytes in 1.1 seconds, verified by a format signature.
DOC to DOCX: 5,462 bytes in 1.1 seconds, verified by a format signature.
DOC to TXT: 268 bytes in 1.1 seconds, verified by reading the internal type and finding a witness word.
DOC to RTF: 4,321 bytes in 1.1 seconds, verified by reading the internal type and finding a witness word.
DOC to ODT: 12,484 bytes in 1.2 seconds, verified by reading the internal type and finding a witness word.
Other ways into HTML, and what they measured
Among the published routes into HTML, DOC is the number 12 largest output of the 12 measured. The witness files differ, so this ranks the probe run and not your document.
XLSX to HTML: 2,269 bytes in 1.1 seconds.
PPTX to HTML: 14,373 bytes in 1.5 seconds.
XLS to HTML: 2,101 bytes in 1.0 seconds.
CSV to HTML: 3,894 bytes in 0.1 seconds.
JSON to HTML: 4,018 bytes in 0.1 seconds.
What we will not pretend about DOC to HTML
A DOC file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A DOC to HTML run that passes 90 seconds is killed, and the LibreOffice process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
LibreOffice is serialised on purpose, one document at a time per worker, because each soffice instance takes 150 to 300 MB on a machine with 3.8 GB. Measured ceiling: 23 DOC conversions in flight and roughly 0.9 a second sustained. Past that the queue answers 429 rather than falling over, which was tested at 32 at once.
Two thirds of the pairs you could imagine in this family do not exist, and it is worth knowing why before you go looking for one. LibreOffice converts inside whichever application opened your file: from a spreadsheet it writes spreadsheets, flat text, HTML and PDF, and it will not write a .docx no matter how it is asked. It returns nothing at all rather than something wrong. Of 113 combinations probed on this engine, 64 came back with a real file and the rest were refused outright.
This pair is one of the 64. It was proved by sending a real DOC file through the live route and reading the HTML that came back: not its extension, its internals, plus a marker word planted in the source and found again in the output. The traits named further up this page come from a second probe that looks for each of them one by one.
One DOC file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
DOC to HTML: what people ask
What actually converts my DOC file to HTML?
LibreOffice does it, 24.2, on a machine we rent and watch. Not a browser trick and not somebody else service: the DOC file is uploaded once, LibreOffice runs once, the HTML comes back, and neither file is kept afterwards.
How long does DOC to HTML take?
On the file the probe used, LibreOffice took 1.1 seconds and wrote 1,626 bytes of HTML. That is one real measurement on one real DOC file, not an average and not a promise about yours: a larger DOC takes longer, and past 90 seconds the run is stopped.
What do I lose going from DOC to HTML?
The one to know about first: Page geometry. HTML has no pages, so margins, page breaks, headers and footers have nowhere to go. This pair turns a document into a web page, not into a picture of one.
Are my speaker notes in the HTML?
No. What lands in the HTML is what was on the slides.
Can I open the HTML without any software?
Yes. Any browser opens it, which is the reason to pick HTML over a DOC: no application, no plugin, no download step for whoever you send it to.
Is DOC to HTML free?
There is a free allowance every month, and one DOC to HTML 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 HTML.