<xsl:template match="tr">
    <xsl:apply-templates select="td|th"/>
</xsl:template>

<xsl:template match="td|th">
  <xsl:variable name="x" select="position()"/>
  <fo:table-cell>
    <xsl:copy-of select="(ancestor::table/colgroup/col)[$x]/@width"/>
    <fo:block>
      <xsl:apply-templates/>
    </fo:block>
  </fo:table-cell>
</xsl:template>

<fo:table-cell width="30%" xmlns:fo="test">
  <fo:block>A header</fo:block>
</fo:table-cell>
<fo:table-cell width="60%" xmlns:fo="test">
  <fo:block>Some content here.</fo:block>
</fo:table-cell>
<fo:table-cell width="10%" xmlns:fo="test">
  <fo:block>Some more here.</fo:block>
</fo:table-cell>

Mike Brown wrote:

Brackets indicate predicates. Think of them as filters, providing everything
to the left of them, for which everything inside them is true.

Got it.

Okay, here's a sample table:

<table>
  <colgroup>
    <col width="30%"/>
  </colgroup>
  <colgroup>
    <col width="60%"/>
    <col width="10%"/>
  </colgroup>
  <tbody>
    <tr>
      <th>A header</th>
      <td>Some content here.</td>
      <td>Some more here.</td>
    </tr>
  </tbody>
</table>

For now, I assume there will always be equal amounts of table cells in a
row as there are <col> elements. The result will look like this:

<fo:table>
  <fo:table-body>
    <fo:table-row>
      <fo:table-cell width="30%">...</fo:table-cell>
      <fo:table-cell width="60%">...</fo:table-cell>
      <fo:table-cell width="10%">...</fo:table-cell>
    </fo:table-row>
  </fo:table-body>
</fo:table>

Here's how the <td> template looks like. The problem is in the long XPath
expressions:

<xsl:template match="td">
<xsl:variable name="x" select="position()"/>
<fo:table-cell xsl:use-attribute-sets="table-padding">
...
<xsl:if test="ancestor::table//col[$x]/attribute::width">
<xsl:attribute name="width">
<xsl:value-of select="ancestor::table//col[$x]/attribute::width"/>
</xsl:attribute>
</xsl:if>
<fo:block>
<xsl:apply-templates/>
</fo:block>
</fo:table-cell>
</xsl:template>
Gustaf