Free Software Directory:Website guidelines
Tabular HTML layout isn't generally a good idea for accessibility, be it for screen readers, text terminals, of magnified screens. They are good to represent raw, statistical, or comparison data. However, if you must use tables, this section describes caveats to take into account.
Avoid tables that make the page width exceed 71 characters
This also includes spaces and non-text areas.
A good way to test this is to use some terminal emulator such as GNOME Terminal with its width set to 80x24 (default) and use a browser such as Lynx. If this number is exceed, Lynx should render the table columns separated by one space, which makes the table confusing to read.
This issue is specially important because one cannot simply recommend everyone to increase their terminal's text width, either because they don't have bigger displays or because they make use of some screen magnifier (in which case they have to keep the terminal fit in the magnifier in order to read it without change the focus back and forth).
Note that the text inside the tables also counts towards the width count, so a long paragraph can easily make the page exceed the limit.
Avoid lists inside table cells
In text browsers which respect the terminal's text width (e.g.: Lynx), the lists are always rendered in the start of the next line, and the table column headers are always separated by a single space. In both changes this makes it difficult to understand where the new cell or heading start.
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