Following explains the source of problem.

commenting out line 85 in tpl-main-page.php [...includes/templates/templates-default/common

<!-- <td id="navColumnOne" class="columnLeft" style="width: <?php echo COLUMN_WIDTH_LEFT; ?>"> -->

but retaining unchanged the CSS selector #navColumnOne {
background-color: #FF0000;
}

removes the red border of the left sidebox, but removes the float layout for everything else

Tomorrow I'll try to work around the issue with a <span> inside the above code line 85.

Whether this issue arises from a server glitch or a programming error I'm not sufficiently acquainted with php to determine. Whatever, I'll be happy to move on.