I highly believe the answer to this question is such:
The priority of widths that will affect the TD is
Table Width
Parent Element Width (and if none, Viewport)
Element(TD) Width.
Hence if the table width is set, the TD's will ALWAYS adjust to the width of the table. However, if the width is unset, the "main" width will be the true width of the viewport. Unless the CSS code states otherwise, this holds true. And only when the total width of the TD's is smaller than that of the viewport, the elemental width will be taken into account.
Edit
Table width will always override TD width.
Stated TD width will only be followed until it exceeds viewport width, and viewport width will be taken as priority.