How can I set the max-width of a table cell using percentages?

According to the definition of max-width in the CSS 2.1 spec, “the effect of 'min-width' and 'max-width' on tables, inline tables, table cells, table columns, and column groups is undefined.” So you cannot directly set max-width on a td element.

If you just want the second column to take up at most 67%, then you can set the width (which is in effect minimum width, for table cells) to 33%, e.g. in the example case

td:first-child { width: 33% ;}

Setting that for both columns won’t work that well, since it tends to make browsers give the columns equal width.

Old question I know, but this is now possible using the css property table-layout: fixed on the table tag. Answer below from this question CSS percentage width and text-overflow in a table cell

This is easily done by using table-layout: fixed, but a little tricky because not many people know about this CSS property.

table {
  width: 100%;
  table-layout: fixed;
}

See it in action at the updated fiddle here: http://jsfiddle.net/Fm5bM/4/

I know this is literally a year later, but I figured I'd share. I was trying to do the same thing and came across this solution that worked for me. We set a max width for the entire table, then worked with the cell sizes for the desired effect.

Put the table in its own div, then set the width, min-width, and/or max-width of the div as desired for the entire table. Then, you can work and set width and min-widths for other cells, and max width for the div effectively working around and backwards to achieve the max width we wanted.

#tablediv {
    width:90%;
    min-width:800px
    max-width:1500px;
}

.tdleft {
    width:20%;
    min-width:200px;
}

<div id="tablediv">
  <table width="100%" border="1">
    <tr>
      <td class="tdleft">Test</td>
      <td>A long string blah blah blah</td>
    </tr>
  </table>
</div>

Admittedly, this does not give you a "max" width of a cell per se, but it does allow some control that might work in-lieu of such an option. Not sure if it will work for your needs. I know it worked for our situation where we want the navigation side in the page to scale up and down to a point but for all the wide screens these days.