I'm writing a little bit of a wiki and going through all of my options for syntax highlighting. Debating between wiki syntax (mediawiki) and markdown + whitelisted tags. I think I would prefer the latter but I think my users will need tables. Why are tables disallowed here on Stackoverflow?
<table> <tr> <td> </td> </tr> </table>
They serve no purpose in a Q&A format. At least I cannot think of a reason I would need to use a table to answer someone's question, or ask one myself.
Plus, you can do this anyway:
cell 1-1 cell 1-2
cell 2-1 cell 2-2
EDIT: So after reading comments on my reply, I see that there may be a few cases where a table could provide a better visual aid. So I'm going to recommend a markdown similar to CSV; I think that's easy enough to type and implement.
I think that it is arbitrary. There could be many uses for them, but it seems that manually aligning fixed-width text is preferred here (which I consider a hack).
Personally, I prefer the BBCode style syntax.
"explicit" means that any intended effect can be expressed in almost any combination, and there are no unintended effects (like in markdown, when one of the many special characters is used). For example, I have no idea how to get this site to display a word in asterisks in a non-fixed font (*word*). Morse code cannot use the underline, because it is also a special character. In BBCode, there is only one special character: [
Additionally, input sanitizing gets much simpler.
Consider that the Wisiwyg javascript editor (WMD) has to render what you are typing in real time
(an important feature wanted by Jeff from the beginning of SO)
Hence, I think dynamic update of table would be way too complex to parse/display, since the HTML translator would have to interpret incomplete table structures as you are typing them.
That means coping with features like colspan, rowspan, incorrect header structures and so on.
So to have a better dynamic preview experience, tables got scratch completely.
There are many cases where tables would be useful: table of data, displaying a matrix, showing the possible outcomes of an algorithm, etc.
I don't think you need something as complex as HTML tables (with rowspan and all), plain CSV would be sufficient for 99% of use-cases, I think. It would also allow the javascript dynamic renderer to do its job easily.
CSV is well-known, light, simple to type and to understand. The only thing needed on top of that would be a start and end tag for CSV data. For example [csv]...[/csv] or ||...||. Here is what it might look like:
[csv]
**XOR**,**true**,**false**
**true**, false, true
**false**, true, false
[/csv]
This would produce a table like this one:
XOR true false
true false true
false true false
(with the first row and the first column in bold characters)
Disallowing tables would be a good idea if your site is built on top of tables and you can't write a regex that is good enough to validate that the users html is syntactically correct, otherwise your layout could be affected.
Even if your site is not table layed out, having two sets of malformed table html in comment posts etc. could lead to your site being defaced.
Three reasons:
Standard Markdown does not support tables. It is intended to be just like e-mail. SO uses standard Markdown, so no tables.
Some Markdown extensions support tables, but they are not compatible between each other, which invalidates the idea of Markdown, because the content becomes dependent on a particular Markdown implementation.
So, the tables can be made only with HTML-inside-Markdown. Which is also not good. I am sure that Markdown2PDF, Markdown2TeX and Markdown2TheNextBigML converters are easy to write. Converting Markdown with embedded HTML to anything but HTML is not trivial. So there is no point to store everything in Markdown (plain text), if (some) embedded HTML is allowed.
Another reason to sanitize all user-submitted HTML is obvious, it is too difficult and expensive to parse properly, and it can break the layout (e.g. <table width="10000" height="10000">).
Finally, there is a huge benefit in a lightweight (pure Markdown) markup: it does not depend on a particular site layout (screen width, paddings, margins, justification, column widths, etc.). So if a SO redesign happens a year from now, the content does not need to be edited (HTML snippets depend on a particular CSS implicitly). Additional bonus: easier to use in third party applications (like mobile phone clients).