Assertions:
If an HR element is used in a table cell and there is no content in the rest of the column, the width of the HR will default to 1 or two pixels. All of the reviewed browsers behave this way (well, except for Mosaic, which seems to automatically impose an ~40 pixel width on such HRs.)

4 column table with plain HR elements in Row 2 Row 1: Col 1 has content. Col 2 has  . Col 3 has a bare space. Col 4 has no content.
Note - Results may be hard to see:

Column 1 






4 column table with HR WIDTH=50 elements in Row 2 Row 1: Col 1 has content. Col 2 has  . Col 3 has a bare space.
Note - Results may be hard to see:

Column 1