HTML is the markup of the web, with native headings and tables. EPUB is a reflowable book split into chapters. pandoc sits between the two, and this page says exactly what happens to an HTML file on the way to becoming an EPUB one.
What runs when an HTML file becomes EPUB
pandoc in two passes, on a machine we rent and watch. Your HTML file is uploaded once, pandoc runs once, the EPUB comes back, and neither file is kept. HTML to EPUB 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 EPUB
The heading hierarchy. pandoc parses the HTML file into its own document tree and writes that tree out as EPUB, so a level two heading is still a level two heading and not merely bold text.
Tables, rebuilt in EPUB 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 EPUB.
Images: pandoc extracts them out of the HTML file into a working directory and re-embeds them in the EPUB, so nothing ends up pointing at a file that no longer exists.
What HTML to EPUB costs you
The presentation layer. pandoc does not translate the HTML file into EPUB, 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.
If a model is going to read the EPUB
Structure is what survives from the HTML file, and structure is what a model needs. HTML in, EPUB out, with the heading tree intact instead of flattened into one long paragraph.
HTML to EPUB, measured rather than promised
Running HTML to EPUB against the real engine with a real file, pandoc wrote 5,027 bytes of EPUB 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 EPUB 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 EPUB 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 EPUB 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 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.
HTML to ODT: 7,629 bytes in 0.1 seconds, verified by a pandoc round trip.
Other ways into EPUB, and what they measured
Among the published routes into EPUB, HTML is the ninth largest output of the 11 measured. The witness files differ, so this ranks the probe run and not your document.
JSON to EPUB: 5,035 bytes in 0.1 seconds.
ODT to EPUB: 5,063 bytes in 0.1 seconds.
RTF to EPUB: 5,033 bytes in 0.1 seconds.
TXT to EPUB: 4,955 bytes in 0.1 seconds.
MOBI to EPUB: 22,644 bytes in 0.6 seconds.
What we will not pretend about HTML to EPUB
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 EPUB 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 EPUB 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 EPUB 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 EPUB: what people ask
What actually converts my HTML file to EPUB?
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 EPUB comes back, and neither file is kept afterwards.
How long does HTML to EPUB take?
On the file the probe used, pandoc took 0.1 seconds and wrote 5,027 bytes of EPUB. 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 EPUB?
The one to know about first: The presentation layer. pandoc does not translate the HTML file into EPUB, 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 EPUB?
Yes. pandoc reads the HTML tables into its own document tree and writes them back out in EPUB syntax, so what you get is a table a machine can parse rather than a picture of one.
Is HTML to EPUB free?
There is a free allowance every month, and one HTML to EPUB 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 EPUB.