CSV is one table and nothing else, with no formulas and no cell formatting. HTML is the markup of the web, with native headings and tables. pandoc sits between the two, and this page says exactly what happens to a CSV file on the way to becoming an HTML one.
What runs when a CSV file becomes HTML
pandoc in two passes, on a machine we rent and watch. Your CSV file is uploaded once, pandoc runs once, the HTML comes back, and neither file is kept. CSV 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 CSV into the HTML
Tables, rebuilt in HTML 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 a CSV file through pandoc rather than through a printer.
Lists from the CSV file, nesting included, plus code blocks, block quotes and links, all re-expressed in HTML.
Images: pandoc extracts them out of the CSV file into a working directory and re-embeds them in the HTML, so nothing ends up pointing at a file that no longer exists.
What CSV to HTML costs you
The presentation layer. pandoc does not translate the CSV file into HTML, 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 CSV and HTML files come from
CSV. Everything writes it and nothing agrees on it. There is an RFC, 4180, and it describes common practice rather than commanding it: quoting, escaping and line endings all vary by producer. A CSV is the lowest common denominator, which is both why it is everywhere and why it surprises people.
If a model is going to read the HTML
Structure is what survives from the CSV file, and structure is what a model needs. CSV in, HTML out, with the heading tree intact instead of flattened into one long paragraph.
CSV to HTML, measured rather than promised
Running CSV to HTML against the real engine with a real file, pandoc wrote 3,894 bytes of HTML in 0.1 seconds, machine otherwise idle. That is one CSV file on one day and not an average, which is why the number is given together with the file that produced it.
The CSV to HTML 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 HTML file has anything inside it.
The same CSV file, sent somewhere else
Other outputs the probe measured out of a CSV 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 CSV witness file.
CSV to PDF: 12,680 bytes in 1.0 seconds, verified by a format signature.
CSV to MD: 114 bytes in 0.1 seconds, verified by a pandoc round trip.
CSV to DOCX: 9,945 bytes in 0.1 seconds, verified by a pandoc round trip.
CSV to EPUB: 4,913 bytes in 0.1 seconds, verified by a pandoc round trip.
CSV to TXT: 114 bytes in under a tenth of a second, verified by reading the text back.
CSV to RTF: 779 bytes in under a tenth of a second, verified by a pandoc round trip.
Other ways into HTML, and what they measured
Among the published routes into HTML, CSV is the eighth largest output of the 12 measured. The witness files differ, so this ranks the probe run and not your document.
JSON to HTML: 4,018 bytes in 0.1 seconds.
ODP to HTML: 12,295 bytes in 1.6 seconds.
ODS to HTML: 2,066 bytes in 1.0 seconds.
ODT to HTML: 4,043 bytes in 0.1 seconds.
PPT to HTML: 27,543 bytes in 1.5 seconds.
What we will not pretend about CSV to HTML
A CSV file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A CSV to HTML 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 CSV to HTML 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 HTML 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 a CSV file goes through LibreOffice or calibre, both of which genuinely can.
One CSV file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
CSV to HTML: what people ask
What actually converts my CSV file to HTML?
pandoc does it, in two passes, on a machine we rent and watch. Not a browser trick and not somebody else service: the CSV file is uploaded once, pandoc runs once, the HTML comes back, and neither file is kept afterwards.
How long does CSV to HTML take?
On the file the probe used, pandoc took 0.1 seconds and wrote 3,894 bytes of HTML. That is one real measurement on one real CSV file, not an average and not a promise about yours: a larger CSV takes longer, and past 60 seconds the run is stopped.
What do I lose going from CSV to HTML?
The one to know about first: The presentation layer. pandoc does not translate the CSV file into HTML, 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 CSV file still tables in the HTML?
Yes. pandoc reads the CSV tables into its own document tree and writes them back out in HTML syntax, so what you get is a table a machine can parse rather than a picture of one.
Is CSV to HTML free?
There is a free allowance every month, and one CSV 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.