See documentation for the HTML 4.0 (XHTML) table model or Mathematical Markup Language (MathML) 2.0 DTD, dependent upon element involved.
In an XHTML table, specifies the desired width of the entire table and is intended for visual user agents. When the value is a percentage value, the value is relative to the user agent’s available horizontal space. In the absence of any width specification, table width is determined by the user agent.
|Data characters||In the XHTML table model, the number that specifies the width of the table|
|Restriction: This attribute may be specified if the element is used.|