The html tags for tables (<table>, <thead>, <tbody>, <th>, <tr>, <td>) are not allowed here because DU uses tables for the page layouts - since anyone using them in a post would stand a good chance of messing up the page formatting those particular tags are ignored in messages by the DU software. However, there are still a few ways to present data in tables on DU.

Perhaps the easiest way is to make a table in some other program (e.g. Excel) and take a screen cap, crop it, put it on an image server and post the image. Of course then no one can copy and paste data from an image. If you want people to have the ability to copy data, you can use CSS to format text in tabular form.

The easier (but not as pretty) CSS way is to use a white-space declaration to preserve spaces on lines and space the info so it lines up vertically. (Use the [br] tag for new lines to avoid the double-spacing introduced by the DU software.) Easiest way to do these "tables" is to use a text editor (Notepad will work) set to a monospace font and line everything up, add [br] tags at the end of lines, then scrunch all the lines together and copy/paste that into a DU post. The basic code should look something like this:

[div class="excerpt" style="margin-left:1em; border:1px solid #bfbfbf; border-radius:0.4615em; box-shadow:-1px -1px 3px #bfbfbf inset;"][div style="font-family:monospace; font-size:1.1em; white-space:pre;"]cell A1    cell A2[br]cell B1    cell B2[br]cell C1    cell C2[/div]