This userscript adds table support to the xkcd forums.
It adds a [table][tr][td][/td][/tr][/table] bbcode that can be used in the editor to add tables. The syntanx works like HTML tables.
It also adds a [csvtable][/csvtable] that creates a table using csv data.
If the viewer has this script, the tables will be displayed using html. If not, they will display by default as aligned monospace text using a code block. Because of this, table cells can only contain plain text, not images, links, or other rich content.
The top row of the table is a header row by default, but that can be disabled with [table={"header":false}]. This also applies to csv tables.
If something doesn't work right, please let me know so I can try to fix it. This has only been tested in the latest version of Chrome, so I don't guarantee it will work in other browsers, though it hopefully should.