* What does missing data look like? The empty string, NaN, 0, 1/1-1970, null, nil, NULL, \0?
* What date format will you need to parse? What does 5/5/12 mean?
* How multiline data has been written? Does it use quotation marks, properly escape those inside multiline strings, or maybe it just expects you to count the delimiter and by the way can delimiters occur inside bare strings?
And let me add my own question here:
what is the actual delimiter? Do you support `,`, `;` and `\t`?
What is the decimal delimiter “.”, “,”?
Most csv users don’t even know they have to be aware of all of these differences.
RFC 4180 provides a (mostly) unambiguous format for writing CSVs, but because it discards the (implied) schema it's useless for reading CSVs that come from other programs. RFC 4180 fields have only one type: text string in US-ASCII encoding. There are no dates, no decimal separators, no letters outside the US-ASCII alphabet, you get nothing! It leaves the option for the MIME type to specify a different text encoding, but that's not part of the resulting file so it's only useful when downloading from the internet.
What are the ambiguities in RFC 4180?
Likewise for whether double quotes (`"`) are allowed in fields (rule 5). This one gets even worse, since the following rule (6) uses double quotes to escape line breaks and commas, but they may not be allowed at all so commas in fields may not be escapable.
It only supports text, not numbers, dates, or any other data, and provides no way to indicate any data type other than text.