[frabjous]

Sorry I should have tested it on a Webkit browser.

It seems to work OK (at least in Chromium 10) if you use constant amounts for whole, half and quarter. For example:

table {
    border-collapse: collapse;
}
td {
    white-space: nowrap;
    padding: 0px;
}
table.whole {
   width: 480px;
   margin: 0px;
}
table.half {
   width: 240px;
   display: inline-block;
}
table.quarter {
   width: 120px;
   display: inline-block;
}
td.fill {
   width: 100%;
   border-bottom: solid 2pt black;
}

Adjust to taste. Unfortunately, that does put some limits on what's possible with regard to font size changes, but I think it should cover all reasonable changes.

I don't have Sigil installed, so I can't test it there.

It would be good to avoid nested tables if possible (though they are quite common on the web). I'll try to think of an alternative, just in case this doesn't work.

(Edit: Oh, yeah, and the comments are there to avoid the line breaks being taken as while space if rendered as HTML rather than XHTML. There's probably an "official" workaround for that, but I'm just making things up as I go, y'know....)