The HTML <th> colspan attribute, used to specify the number of columns a header cell should span, is widely supported across all modern web browsers. This includes: Google Chrome, Mozilla Firefox, Microsoft Edge, Apple Safari, and Opera.

Support for the colspan attribute in <th> elements has been present in these browsers since their early versions, ensuring consistent rendering of table layouts across different platforms. This attribute functions identically for both <th> (table header) and <td> (table data) elements.