How to Enable the Automatic Table Layout
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The DITA specification indicates that tables should have a fixed layout. However, you can get a more optimal arrangement of the cells using the automatic layout. In your customization CSS, add:

*[class~="topic/tgroup"] {
    table-layout:auto !important;
}
If you want to control which table you want this layout, you can set the @outputclass attribute on the <table> element:
<table outputclass='auto_tbl'> ... </table>
Then, in the CSS, use a rule that matches the @outputclass:
*[class~="topic/table"][outputclass='auto_tbl'] > *[class~="topic/tgroup"] {
    table-layout:auto !important;
}
Important: Make sure the tables have no column width specified.