Wikipedia:Manual of Style/Accessibility/Data tables tutorial

Note that each column header is repeated when reading every row, so an abbreviation could be added to long headers using the abbr="..." attribute, for example:

{|
|+ [caption text]
|-
! abbr="Wikipedian" | Wikipedia editor
! abbr="Edits"      | Number of edits
! Last edit
! abbr="Donations"  | Donations (US$)
|-
...

In this example all column headers have an abbreviation except the column with the date of the last edit, which is short enough.

Row or column headers should not use colspan/rowspan to span several rows or columns – unless sub-headers are provided. Tables should not contain more than two levels of headers (which means 1) headers 2) sub-headers; but no 3) sub-sub headers).

Images or colors inside a table should meet the requirements for images and color.