Colspan all columns

Just use this:

colspan="100%"

It works on Firefox 3.6, IE 7 and Opera 11! (and I guess on others, I couldn't try)

Warning: as mentioned in the comments below this is actually the same as colspan="100". Hence, this solution will break for tables with css table-layout: fixed, or more than 100 columns.

I have IE 7.0, Firefox 3.0 and Chrome 1.0

The colspan="0" attribute in a TD is NOT spanning across all TDs in any of the above browsers.

Maybe not recommended as proper markup practice, but if you give a higher colspan value than the total possible no. of columns in other rows, then the TD would span all the columns.

This does NOT work when the table-layout CSS property is set to fixed.

Once again, this is not the perfect solution but seems to work in the above mentioned 3 browser versions when the table-layout CSS property is automatic. Hope this helps.

As a partial answer, here's a few points about colspan="0", which was mentioned in the question.

tl;dr version:

colspan="0" doesn't work in any browser whatsoever. W3Schools is wrong (as usual). HTML 4 said that colspan="0" should cause a column to span the whole table, but nobody implemented this and it was removed from the spec after HTML 4.

Some more detail and evidence:

• All major browsers treat it as equivalent to colspan="1".

  Here's a demo showing this; try it on any browser you like.

  td {
    border: 1px solid black;
  }

  <table>
    <tr>
      <td>ay</td>
      <td>bee</td>
      <td>see</td>
    </tr>
    <tr>
      <td colspan="0">colspan="0"</td>
    </tr>
    <tr>
      <td colspan="1">colspan="1"</td>
    </tr>
    <tr>
      <td colspan="3">colspan="3"</td>
    </tr>
    <tr>
      <td colspan="1000">colspan="1000"</td>
    </tr>
  </table>

• The HTML 4 spec (now old and outdated, but current back when this question was asked) did indeed say that colspan="0" should be treated as spanning all columns:

  The value zero ("0") means that the cell spans all columns from the current column to the last column of the column group (COLGROUP) in which the cell is defined.

  However, most browsers never implemented this.

• HTML 5.0 (made a candidate recommendation back in 2012), the WhatWG HTML living standard (the dominant standard today), and the latest W3 HTML 5 spec all do not contain the wording quoted from HTML 4 above, and unanimously agree that a colspan of 0 is not allowed, with this wording which appears in all three specs:

  The td and th elements may have a colspan content attribute specified, whose value must be a valid non-negative integer greater than zero ...

  Sources:

  • https://www.w3.org/TR/html50/tabular-data.html#attributes-common-to-td-and-th-elements
  • https://html.spec.whatwg.org/multipage/tables.html#attributes-common-to-td-and-th-elements
  • https://www.w3.org/TR/html53/tabular-data.html#attributes-common-to-td-and-th-elements

• The following claims from the W3Schools page linked to in the question are - at least nowadays - completely false:

  Only Firefox supports colspan="0", which has a special meaning ... [It] tells the browser to span the cell to the last column of the column group (colgroup)

  and

  Differences Between HTML 4.01 and HTML5

  NONE.

  If you're not already aware that W3Schools is generally held in contempt by web developers for its frequent inaccuracies, consider this a lesson in why.

For IE 6, you'll want to equal colspan to the number of columns in your table. If you have 5 columns, then you'll want: colspan="5".

The reason is that IE handles colspans differently, it uses the HTML 3.2 specification:

IE implements the HTML 3.2 definition, it sets colspan=0 as colspan=1.

The bug is well documented.

Below is a concise es6 solution (similar to Rainbabba's answer but without the jQuery).

Array.from(document.querySelectorAll('[data-colspan-max]')).forEach(td => {
    let table = td;
    while (table && table.nodeName !== 'TABLE') table = table.parentNode;
    td.colSpan = Array.from(table.querySelector('tr').children).reduce((acc, child) => acc + child.colSpan, 0);
});

html {
  font-family: Verdana;
}
tr > * {
  padding: 1rem;
  box-shadow: 0 0 8px gray inset;
}

<table>
<thead>
  <tr>
    <th>Header 1</th>
    <th>Header 2</th>
    <th>Header 3</th>
    <th>Header 4</th>
    <th>Header 5</th>
    <th>Header 6</th>
  </tr>
</thead>
<tbod><tr>
  <td data-colspan-max>td will be set to full width</td>
</tr></tbod>
</table>

Simply set colspan to the number of columns in the table.

All other "shortcuts" have pitfalls.

The best thing to do is set the colspan to the correct number to begin with. If your table has 5 columns, set it to colspan="5" That is the only way that will work in all scenarios. No, it's not an outdated solution or only recommended for IE6 or anything -- that's literally the best way to handle this.

I wouldn't recommend using Javascript to solve this unless the number of columns changes during runtime.

If the number of columns is variable, then you'll need to calculate the number of columns so that you can populate the colspan. If you have a variable number of columns, whatever is generating the table should be able to be adapted to also calculate the number of columns the table has.

As other answers have mentioned, if your table is not set to table-layout: fixed, you can also just set colspan to a really large number. But I find this solution messy, and it can be a headache if you come back later and decide it should be a fixed table layout. Better just to do it correctly the first time.

A CSS solution would be ideal, but I was unable to find one, so here is a JavaScript solution: for a tr element with a given class, maximize it by selecting a full row, counting its td elements and their colSpan attributes, and just setting the widened row with el.colSpan = newcolspan;. Like so...

var headertablerows = document.getElementsByClassName('max-col-span');

[].forEach.call(headertablerows, function (headertablerow) {
    var colspan = 0;
    [].forEach.call(headertablerow.nextElementSibling.children, function (child) {
        colspan += child.colSpan ? parseInt(child.colSpan, 10) : 1;
    });
    
    headertablerow.children[0].colSpan = colspan;
});

html {
  font-family: Verdana;
}
tr > * {
  padding: 1rem;
  box-shadow: 0 0 8px gray inset;
}

  <table>
    <tr class="max-col-span">
      <td>1 - max width
      </td>
    </tr>
    <tr>
      <td>2 - no colspan
      </td>
      <td colspan="2">3 - colspan is 2
      </td>
    </tr>
  </table>

You may need to adjust this if you're using table headers, but this should give a proof-of-concept approach that uses 100% pure JavaScript.